diff --git a/synthesis/build_defs.bzl b/synthesis/build_defs.bzl
index f20b427e..a97d70ed 100644
--- a/synthesis/build_defs.bzl
+++ b/synthesis/build_defs.bzl
@@ -130,6 +130,8 @@ def _synthesize_design_impl(ctx):
     args.add("-Q")  # Don't print header
     args.add_all("-l", [log_file])  # put output in log file
     args.add_all("-c", [synth_tcl])  # run synthesis tcl script
+    if ctx.attr.extra_tcl_command:
+        args.add("-p", ctx.attr.extra_tcl_command)
 
     dont_use_args = ""
     or_config = ctx.attr.standard_cells[StandardCellInfo].open_road_configuration
@@ -362,6 +364,9 @@ synthesize_rtl = rule(
             allow_single_file = True,
             doc = "Tcl synthesis script compatible with the environment-variable API of synth.tcl",
         ),
+        "extra_tcl_command": attr.string(
+            default = "",
+        ),
         "target_clock_period_pico_seconds": attr.int(
             doc = "target clock period in picoseconds",
         ),