diff --git a/coupler/ensemble_manager.F90 b/coupler/ensemble_manager.F90
index e31105ddb8..1057804cae 100644
--- a/coupler/ensemble_manager.F90
+++ b/coupler/ensemble_manager.F90
@@ -49,6 +49,26 @@ module ensemble_manager_mod
 
   integer :: ensemble_size = 1
   integer :: ensemble_id = 1
+!> @brief Starting index for ensemble numbering in I/O filenames.
+!!
+!! Defines an offset applied to the local ensemble_id when constructing
+!! filename appendices via fms2_io_set_filename_appendix.
+!!
+!! The effective ensemble index used in filenames is:
+!!   global_ens = starting_ensemble_id + ensemble_id - 1
+!!
+!! For example:
+!!   ensemble_size = 2
+!!   starting_ensemble_id = 10
+!! results in filenames using ens_10 and ens_11.
+!!
+!! @note This parameter affects only I/O filename construction (e.g.,
+!! restart and diagnostic files). It does not affect ensemble-specific
+!! input selection such as data_override, which uses explicit filenames
+!! or data_table.ens_XX.yaml.
+!!
+!! @default 1
+  integer :: starting_ensemble_id = 1
   integer :: pe, total_npes_pm=0,ocean_npes_pm=0,atmos_npes_pm=0
   integer :: land_npes_pm=0,ice_npes_pm=0
 
@@ -75,7 +95,11 @@ subroutine ensemble_manager_init()
 
     integer :: i, io_status, npes, ierr
 
-    namelist /ensemble_nml/ ensemble_size
+    !> @namelist ensemble_nml
+    !! @param ensemble_size Number of ensemble members.
+    !! @param starting_ensemble_id Offset applied to ensemble_id when
+    !!        constructing I/O filename appendices.
+    namelist /ensemble_nml/ ensemble_size, starting_ensemble_id
 
     read (input_nml_file, ensemble_nml, iostat=io_status)
     ierr = check_nml_error(io_status, 'ensemble_nml')
@@ -85,6 +109,8 @@ subroutine ensemble_manager_init()
     if(ensemble_size > max_ensemble_size)  call mpp_error(FATAL, &
        'ensemble_manager_mod: ensemble_nml variable ensemble_size should be no larger than MAX_ENSEMBLE_SIZE, '// &
        'change ensemble_size or increase MAX_ENSEMBLE_SIZE')
+    if(starting_ensemble_id < 1)  call mpp_error(FATAL, &
+       'ensemble_manager_mod: ensemble_nml variable starting_ensemble_id must be a positive integer')
 
     pe = mpp_pe()
     npes = mpp_npes()
@@ -238,7 +264,7 @@ subroutine ensemble_pelist_setup(concurrent, atmos_npes, ocean_npes, land_npes,
     integer           :: atmos_pe_start, atmos_pe_end, ocean_pe_start, ocean_pe_end
     integer           :: land_pe_start, land_pe_end, ice_pe_start, ice_pe_end
     character(len=10) :: pelist_name, text
-    integer           :: npes, n, m ,i
+    integer           :: npes, n, m ,i, global_ens
 
     npes = total_npes_pm
 
@@ -401,7 +427,8 @@ subroutine ensemble_pelist_setup(concurrent, atmos_npes, ocean_npes, land_npes,
     !can be used for non-ensemble experiments
     !
     if (ensemble_size > 1) then
-       write( text,'(a,i2.2)' ) 'ens_', ensemble_id
+       global_ens = starting_ensemble_id + ensemble_id - 1
+       write( text,'(a,i2.2)' ) 'ens_', global_ens
        !Append ensemble_id to the restart filenames
        call fms2_io_set_filename_appendix(trim(text))
     endif