diff --git a/docs/CONTRIBUTORS b/docs/CONTRIBUTORS
index 72f1e9dc7..af921dad7 100644
--- a/docs/CONTRIBUTORS
+++ b/docs/CONTRIBUTORS
@@ -224,6 +224,7 @@ Marshal Qiao
 Martijn Wobbes
 Martin Schmidt
 Martin Stadler
+Martin Velay
 Mateusz Gancarz
 Matt Stroud
 Matthew Ballance
diff --git a/include/verilated.cpp b/include/verilated.cpp
index ecfb98881..f1f7af607 100644
--- a/include/verilated.cpp
+++ b/include/verilated.cpp
@@ -1695,7 +1695,8 @@ IData _vl_vsscanf(FILE* fp,  // If a fscanf
                     _vl_vsss_read_str(fp, floc, fromp, fstr,
                                       std::back_insert_iterator<std::string>{t_tmp},
                                       "0123456789+-xXzZ?_");
-                    if (!t_tmp[0]) goto done;
+                    t_tmp.erase(std::remove(t_tmp.begin(), t_tmp.end(), '_'), t_tmp.end());
+                    if (t_tmp.empty()) goto done;
                     if (formatAttr == VL_VFORMATATTR_SIGNED) {
                         QData ld = 0;
                         std::sscanf(t_tmp.c_str(), "%30" PRIu64, &ld);
@@ -2401,8 +2402,10 @@ IData VL_VALUEPLUSARGS_INW(int rbits, const std::string& ld, WDataOutP rwp) VL_M
     VL_ZERO_W(rbits, rwp);
     switch (std::tolower(fmt)) {
     case 'd': {
+        std::string digits{dp};
+        digits.erase(std::remove(digits.begin(), digits.end(), '_'), digits.end());
         int64_t lld = 0;
-        std::sscanf(dp, "%30" PRId64, &lld);
+        std::sscanf(digits.c_str(), "%30" PRId64, &lld);
         VL_SET_WQ(rwp, lld);
         break;
     }
diff --git a/test_regress/t/t_sys_plusargs.py b/test_regress/t/t_sys_plusargs.py
index fa6e7da58..3b0ce06a9 100755
--- a/test_regress/t/t_sys_plusargs.py
+++ b/test_regress/t/t_sys_plusargs.py
@@ -13,6 +13,10 @@ test.scenarios('simulator')
 
 test.compile(v_flags2=['-v', 't/t_flag_libinc.v'])
 
-test.execute(all_run_flags=['+PLUS +INT=1234 +STRSTR +REAL=1.2345 +IP%P101'])
+test.execute(all_run_flags=[
+    '+PLUS +INT=1234 +STRSTR +REAL=1.2345 +IP%P101',
+    '+DECIMAL=300_000_000_000 +NEGATIVE=-300_000_000_000',
+    '+LEADING=_1_234 +TRAILING=1_234_ +UNDERSCORE=_',
+])
 
 test.passes()
diff --git a/test_regress/t/t_sys_plusargs.v b/test_regress/t/t_sys_plusargs.v
index 45efb4c88..fd0c1cdc4 100644
--- a/test_regress/t/t_sys_plusargs.v
+++ b/test_regress/t/t_sys_plusargs.v
@@ -4,6 +4,11 @@
 // SPDX-FileCopyrightText: 2009 Wilson Snyder
 // SPDX-License-Identifier: CC0-1.0
 
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d\n", `__FILE__,`__LINE__, (gotv), (expv)); `stop; end while(0);
+// verilog_format: on
+
 module t;
 
   integer p_i;  // signal type IData
@@ -15,6 +20,8 @@ module t;
   reg [7*8:1] p_in;
   string sv_in;
   integer unread;  // never read
+  longint signed p_decimal;
+  int code;
 
   initial begin
     if ($test$plusargs("PLUS") !== 1) $stop;
@@ -128,6 +135,23 @@ module t;
     if (p_i !== 32'd1234) $stop;
     if (p_r !== 1.2345) $stop;
 
+    code = $value$plusargs("DECIMAL=%d", p_decimal);
+    `checkd(code, 1);
+    `checkd(p_decimal, 64'sd300_000_000_000);
+    code = $value$plusargs("NEGATIVE=%d", p_decimal);
+    `checkd(code, 1);
+    `checkd(p_decimal, -64'sd300_000_000_000);
+    code = $value$plusargs("LEADING=%0d", p_decimal);
+    `checkd(code, 1);
+    `checkd(p_decimal, 1_234);
+    code = $value$plusargs("TRAILING=%d", p_decimal);
+    `checkd(code, 1);
+    `checkd(p_decimal, 1_234);
+    // A matching plusarg returns 1 even when its value contains no digits.
+    code = $value$plusargs("UNDERSCORE=%d", p_decimal);
+    `checkd(code, 1);
+    `checkd(p_decimal, 0);
+
     $write("*-* All Finished *-*\n");
     $finish;
   end
diff --git a/test_regress/t/t_sys_sscanf.v b/test_regress/t/t_sys_sscanf.v
index 3d32dd0ca..dd88e62fa 100644
--- a/test_regress/t/t_sys_sscanf.v
+++ b/test_regress/t/t_sys_sscanf.v
@@ -6,6 +6,7 @@
 
 // verilog_format: off
 `define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d\n", `__FILE__,`__LINE__, (gotv), (expv)); `stop; end while(0);
 `define checkh(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got='h%x exp='h%x\n", `__FILE__,`__LINE__, (gotv), (expv)); `stop; end while(0);
 `define checks(gotv,expv) do if ((gotv) != (expv)) begin $write("%%Error: %s:%0d:  got='%s' exp='%s'\n", `__FILE__,`__LINE__, (gotv), (expv)); `stop; end while(0);
 // verilog_format: on
@@ -18,6 +19,9 @@ module t;
   int unsigned idx;
   logic [XLEN-1:0] val;
   int code;
+  longint signed scanned;
+  real scanned_real;
+  byte next_char;
 
   reg [255:0] line;
   reg [63:0] token;
@@ -49,6 +53,55 @@ module t;
     code = $sscanf("", "%d", idx);
     `checkh(code, -1);
 
+    // Decimal digit separators, including leading and trailing underscores.
+    code = $sscanf("10_000_000", "%d", scanned);
+    `checkd(code, 1);
+    `checkd(scanned, 10_000_000);
+    code = $sscanf("-10_000_000", "%d", scanned);
+    `checkd(code, 1);
+    `checkd(scanned, -10_000_000);
+    code = $sscanf("1_234", "%d", idx);
+    `checkd(code, 1);
+    `checkd(idx, 1_234);
+    code = $sscanf("_1_234", "%d", scanned);
+    `checkd(code, 1);
+    `checkd(scanned, 1_234);
+    code = $sscanf("1_234_", "%d", scanned);
+    `checkd(code, 1);
+    `checkd(scanned, 1_234);
+
+    // An underscore-only field must not assign a value or count as a conversion.
+    scanned = 99;
+    code = $sscanf("_", "%d", scanned);
+    `checkd(code, -1);
+    `checkd(scanned, 99);
+    code = $sscanf("_ ", "%d", scanned);
+    `checkd(code, 0);
+    `checkd(scanned, 99);
+    code = $sscanf("7 _", "%d %d", idx, scanned);
+    `checkd(code, 1);
+    `checkd(idx, 7);
+    `checkd(scanned, 99);
+
+    // IEEE 1800-2023 Table 21-7 excludes underscores from floating-point fields.
+    // Leave the underscore unread so the following conversion can consume it.
+    code = $sscanf("1_234.5", "%f%c", scanned_real, next_char);
+    `checkd(code, 2);
+    `checkh($realtobits(scanned_real), $realtobits(1.0));
+    `checkd(next_char, 8'h5f);
+    code = $sscanf("1.25_e2", "%e%c", scanned_real, next_char);
+    `checkd(code, 2);
+    `checkh($realtobits(scanned_real), $realtobits(1.25));
+    `checkd(next_char, 8'h5f);
+    code = $sscanf("1.25e1_0", "%g%c", scanned_real, next_char);
+    `checkd(code, 2);
+    `checkh($realtobits(scanned_real), $realtobits(12.5));
+    `checkd(next_char, 8'h5f);
+    scanned_real = 99.0;
+    code = $sscanf("_", "%f", scanned_real);
+    `checkd(code, 0);
+    `checkh($realtobits(scanned_real), $realtobits(99.0));
+
     $finish;
   end