diff --git a/main/CMakeLists.txt b/main/CMakeLists.txt
index a431a2b00ad0b..113f15e478299 100644
--- a/main/CMakeLists.txt
+++ b/main/CMakeLists.txt
@@ -14,7 +14,7 @@ if(NOT WIN32)
 endif()
 ROOT_EXECUTABLE(root.exe rmain.cxx LIBRARIES Core Rint)
 ROOT_EXECUTABLE(proofserv.exe pmain.cxx LIBRARIES Core MathCore)
-ROOT_EXECUTABLE(hadd hadd.cxx LIBRARIES Core RIO Net Hist Graf Graf3d Gpad Tree Matrix MathCore Thread)
+ROOT_EXECUTABLE(hadd hadd.cxx LIBRARIES Core RIO Net Hist Graf Graf3d Gpad Tree Matrix MathCore MultiProc)
 ROOT_EXECUTABLE(rootnb.exe nbmain.cxx LIBRARIES Core)
 
 if(fortran AND CMAKE_Fortran_COMPILER)
diff --git a/main/src/hadd.cxx b/main/src/hadd.cxx
index 812fa89a41393..bb06a11afea50 100644
--- a/main/src/hadd.cxx
+++ b/main/src/hadd.cxx
@@ -79,11 +79,13 @@
 #include "Riostream.h"
 #include "TClass.h"
 #include "TSystem.h"
+#include "TUUID.h"
 #include "ROOT/StringConv.hxx"
 #include <stdlib.h>
 #include <climits>
 
 #include "TFileMerger.h"
+#include "ROOT/TProcessExecutor.hxx"
 
 ////////////////////////////////////////////////////////////////////////////////
 
@@ -104,6 +106,13 @@ int main( int argc, char **argv )
       std::cout << "If the option -O is used, when merging TTree, the basket size is re-optimized" <<std::endl;
       std::cout << "If the option -v is used, explicitly set the verbosity level;\n"\
                    "   0 request no output, 99 is the default" <<std::endl;
+      std::cout << "If the option -j is used, the execution will be parallelized in multiple processes\n" << std::endl;
+      std::cout << "If the option -dbg is used, the execution will be parallelized in multiple processes in debug mode."
+                   " This will not delete the partial files stored in the working directory\n"
+                << std::endl;
+      std::cout << "If the option -d is used, the partial multiprocess execution will be carried out in the specified "
+                   "directory\n"
+                << std::endl;
       std::cout << "If the option -n is used, hadd will open at most 'maxopenedfiles' at once, use 0\n"
                    "   to request to use the system maximum." << std::endl;
       std::cout << "If the option -cachesize is used, hadd will resize (or disable if 0) the\n"
@@ -139,10 +148,15 @@ int main( int argc, char **argv )
    Bool_t noTrees = kFALSE;
    Bool_t keepCompressionAsIs = kFALSE;
    Bool_t useFirstInputCompression = kFALSE;
+   Bool_t multiproc = kFALSE;
+   Bool_t debug = kFALSE;
    Int_t maxopenedfiles = 0;
    Int_t verbosity = 99;
    TString cacheSize;
-
+   SysInfo_t s;
+   gSystem->GetSysInfo(&s);
+   auto nProcesses = s.fCpus;
+   auto workingDir = gSystem->TempDirectory();
    int outputPlace = 0;
    int ffirst = 2;
    Int_t newcomp = -1;
@@ -162,6 +176,53 @@ int main( int argc, char **argv )
       } else if ( strcmp(argv[a],"-O") == 0 ) {
          reoptimize = kTRUE;
          ++ffirst;
+      } else if (strcmp(argv[a], "-dbg") == 0) {
+         debug = kTRUE;
+         verbosity = kTRUE;
+         ++ffirst;
+      } else if (strcmp(argv[a], "-d") == 0) {
+         if (a + 1 != argc && argv[a + 1][0] != '-') {
+            if (gSystem->AccessPathName(argv[a + 1])) {
+               std::cerr << "Error: could not access the directory specified: " << argv[a + 1]
+                         << ". We will use the system's temporal directory.\n";
+            } else {
+               workingDir = argv[a + 1];
+            }
+            ++a;
+            ++ffirst;
+         } else {
+            std::cout << "-d: no directory specified.  We will use the system's temporal directory.\n";
+         }
+         ++ffirst;
+      } else if (strcmp(argv[a], "-j") == 0) {
+         // If the number of processes is not specified, use the default.
+         if (a + 1 != argc && argv[a + 1][0] != '-') {
+            // number of processes specified
+            Long_t request = 1;
+            for (char *c = argv[a + 1]; *c != '\0'; ++c) {
+               if (!isdigit(*c)) {
+                  // Wrong number of Processes. Use the default:
+                  std::cerr << "Error: could not parse the number of processes to run in parallel passed after -j: "
+                            << argv[a + 1] << ". We will use the system maximum.\n";
+                  request = 0;
+                  break;
+               }
+            }
+            if (request == 1) {
+               request = strtol(argv[a + 1], 0, 10);
+               if (request < kMaxLong && request >= 0) {
+                  nProcesses = (Int_t)request;
+                  ++a;
+                  ++ffirst;
+                  std::cout << "Parallelizing  with " << nProcesses << " processes.\n";
+               } else {
+                  std::cerr << "Error: could not parse the number of processes to use passed after -j: " << argv[a + 1]
+                            << ". We will use the default value (number of logical cores).\n";
+               }
+            }
+         }
+         multiproc = kTRUE;
+         ++ffirst;
       } else if ( strcmp(argv[a],"-cachesize=") == 0 ) {
          int size;
          static const size_t arglen = strlen("-cachesize=");
@@ -308,11 +369,11 @@ int main( int argc, char **argv )
       std::cout << "hadd Target file: " << targetname << std::endl;
    }
 
-   TFileMerger merger(kFALSE,kFALSE);
-   merger.SetMsgPrefix("hadd");
-   merger.SetPrintLevel(verbosity - 1);
+   TFileMerger fileMerger(kFALSE, kFALSE);
+   fileMerger.SetMsgPrefix("hadd");
+   fileMerger.SetPrintLevel(verbosity - 1);
    if (maxopenedfiles > 0) {
-      merger.SetMaxOpenedFiles(maxopenedfiles);
+      fileMerger.SetMaxOpenedFiles(maxopenedfiles);
    }
    if (newcomp == -1) {
       if (useFirstInputCompression || keepCompressionAsIs) {
@@ -348,62 +409,135 @@ int main( int argc, char **argv )
          std::cout << "hadd compression setting for all ouput: " << newcomp << '\n';
    }
    if (append) {
-      if (!merger.OutputFile(targetname,"UPDATE",newcomp)) {
+      if (!fileMerger.OutputFile(targetname, "UPDATE", newcomp)) {
          std::cerr << "hadd error opening target file for update :" << argv[ffirst-1] << "." << std::endl;
          exit(2);
       }
-   } else if (!merger.OutputFile(targetname,force,newcomp) ) {
+   } else if (!fileMerger.OutputFile(targetname, force, newcomp)) {
       std::cerr << "hadd error opening target file (does " << argv[ffirst-1] << " exist?)." << std::endl;
       if (!force) std::cerr << "Pass \"-f\" argument to force re-creation of output file." << std::endl;
       exit(1);
    }
 
+   auto filesToProcess = argc - ffirst;
+   auto step = (filesToProcess + nProcesses - 1) / nProcesses;
+   if (multiproc && step < 3) {
+      // At least 3 files per process
+      step = 3;
+      nProcesses = (filesToProcess + step - 1) / step;
+      std::cout << "Each process should handle at least 3 files for efficiency.";
+      std::cout << " Setting the number of processes to: " << nProcesses << std::endl;
+   }
+   std::vector<std::string> partialFiles;
+
+   if (multiproc) {
+      auto uuid = TUUID();
+      auto partialTail = uuid.AsString();
+      for (auto i = 0; (i * step) < filesToProcess; i++) {
+         std::stringstream buffer;
+         buffer << workingDir << "/partial" << i << "_" << partialTail << ".root";
+         partialFiles.emplace_back(buffer.str());
+      }
+   }
 
-   for ( int i = ffirst; i < argc; i++ ) {
-      if (argv[i] && argv[i][0]=='@') {
-         std::ifstream indirect_file(argv[i]+1);
-         if( ! indirect_file.is_open() ) {
-            std::cerr<< "hadd could not open indirect file " << (argv[i]+1) << std::endl;
-            return 1;
+   auto mergeFiles = [&](TFileMerger &merger) {
+      if (reoptimize) {
+         merger.SetFastMethod(kFALSE);
+      } else {
+         if (!keepCompressionAsIs && merger.HasCompressionChange()) {
+            // Don't warn if the user any request re-optimization.
+            std::cout << "hadd Sources and Target have different compression levels" << std::endl;
+            std::cout << "hadd merging will be slower" << std::endl;
          }
-         while( indirect_file ){
-            std::string line;
-            if( std::getline(indirect_file, line) && line.length() &&  !merger.AddFile(line.c_str()) ) {
-               return 1;
+      }
+      merger.SetNotrees(noTrees);
+      merger.SetMergeOptions(cacheSize);
+      Bool_t status;
+      if (append)
+         status = merger.PartialMerge(TFileMerger::kIncremental | TFileMerger::kAll);
+      else
+         status = merger.Merge();
+      return status;
+   };
+
+   auto sequentialMerge = [&](TFileMerger &merger, int start, int nFiles) {
+
+      for (auto i = start; i < (start + nFiles) && i < argc; i++) {
+         if (argv[i] && argv[i][0] == '@') {
+            std::ifstream indirect_file(argv[i] + 1);
+            if (!indirect_file.is_open()) {
+               std::cerr << "hadd could not open indirect file " << (argv[i] + 1) << std::endl;
+               return kFALSE;
             }
+            while (indirect_file) {
+               std::string line;
+               if (std::getline(indirect_file, line) && line.length() && !merger.AddFile(line.c_str())) {
+                  return kFALSE;
+               }
+            }
+         } else if (!merger.AddFile(argv[i])) {
+            if (skip_errors) {
+               std::cerr << "hadd skipping file with error: " << argv[i] << std::endl;
+            } else {
+               std::cerr << "hadd exiting due to error in " << argv[i] << std::endl;
+            }
+            return kFALSE;
          }
-      } else if( ! merger.AddFile(argv[i]) ) {
-         if ( skip_errors ) {
-            std::cerr << "hadd skipping file with error: " << argv[i] << std::endl;
-         } else {
-            std::cerr << "hadd exiting due to error in " << argv[i] << std::endl;
-            return 1;
+      }
+      return mergeFiles(merger);
+   };
+
+   auto parallelMerge = [&](int start) {
+      TFileMerger mergerP(kFALSE, kFALSE);
+      mergerP.SetMsgPrefix("hadd");
+      mergerP.SetPrintLevel(verbosity - 1);
+      if (maxopenedfiles > 0) {
+         mergerP.SetMaxOpenedFiles(maxopenedfiles / nProcesses);
+      }
+      if (!mergerP.OutputFile(partialFiles[(start - ffirst) / step].c_str(), newcomp)) {
+         std::cerr << "hadd error opening target partial file" << std::endl;
+         exit(1);
+      }
+      return sequentialMerge(mergerP, start, step);
+   };
+
+   auto reductionFunc = [&]() {
+      for (auto pf : partialFiles) {
+         fileMerger.AddFile(pf.c_str());
+      }
+      return mergeFiles(fileMerger);
+   };
+
+   Bool_t status;
+
+   if (multiproc) {
+      ROOT::TProcessExecutor p(nProcesses);
+      auto res = p.Map(parallelMerge, ROOT::TSeqI(ffirst, argc, step));
+      status = std::accumulate(res.begin(), res.end(), 0U) == partialFiles.size();
+      if (status) {
+         status = reductionFunc();
+      } else {
+         std::cout << "hadd failed at the parallel stage" << std::endl;
+      }
+      if (!debug) {
+         for (auto pf : partialFiles) {
+            gSystem->Unlink(pf.c_str());
          }
       }
-   }
-   if (reoptimize) {
-      merger.SetFastMethod(kFALSE);
    } else {
-      if (!keepCompressionAsIs && merger.HasCompressionChange()) {
-         // Don't warn if the user any request re-optimization.
-         std::cout <<"hadd Sources and Target have different compression levels"<<std::endl;
-         std::cout <<"hadd merging will be slower"<<std::endl;
-      }
+      status = sequentialMerge(fileMerger, ffirst, filesToProcess);
    }
-   merger.SetNotrees(noTrees);
-   merger.SetMergeOptions(cacheSize);
-   Bool_t status;
-   if (append) status = merger.PartialMerge(TFileMerger::kIncremental | TFileMerger::kAll);
-   else status = merger.Merge();
 
    if (status) {
       if (verbosity == 1) {
-         std::cout << "hadd merged " << merger.GetMergeList()->GetEntries() << " input files in " << targetname << ".\n";
+         std::cout << "hadd merged " << fileMerger.GetMergeList()->GetEntries() << " input files in " << targetname
+                   << ".\n";
       }
       return 0;
    } else {
       if (verbosity == 1) {
-         std::cout << "hadd failure during the merge of " << merger.GetMergeList()->GetEntries() << " input files in " << targetname << ".\n";
+         std::cout << "hadd failure during the merge of " << fileMerger.GetMergeList()->GetEntries()
+                   << " input files in " << targetname << ".\n";
       }
       return 1;
    }