diff --git a/amaranth/vendor/_lattice_ecp5.py b/amaranth/vendor/_lattice_ecp5.py
index 10c8e58..5bb2199 100644
--- a/amaranth/vendor/_lattice_ecp5.py
+++ b/amaranth/vendor/_lattice_ecp5.py
@@ -234,6 +234,7 @@ class LatticeECP5Platform(TemplatedPlatform):
             {{get_override("add_preferences")|default("# (add_preferences placeholder)")}}
         """,
         "{{name}}.sdc": r"""
+            set_hierarchy_separator {/}
             {% for net_signal, port_signal, frequency in platform.iter_clock_constraints() -%}
                 {% if port_signal is not none -%}
                     create_clock -name {{port_signal.name|tcl_escape}} -period {{1000000000/frequency}} [get_ports {{port_signal.name|tcl_escape}}]
diff --git a/amaranth/vendor/_lattice_ice40.py b/amaranth/vendor/_lattice_ice40.py
index 0894025..9e76bc9 100644
--- a/amaranth/vendor/_lattice_ice40.py
+++ b/amaranth/vendor/_lattice_ice40.py
@@ -236,6 +236,7 @@ class LatticeICE40Platform(TemplatedPlatform):
         """,
         "{{name}}.sdc": r"""
             # {{autogenerated}}
+            set_hierarchy_separator {/}
             {% for net_signal, port_signal, frequency in platform.iter_clock_constraints() -%}
                 {% if port_signal is not none -%}
                     create_clock -name {{port_signal.name|tcl_escape}} -period {{1000000000/frequency}} [get_ports {{port_signal.name|tcl_escape}}]
diff --git a/amaranth/vendor/_lattice_machxo_2_3l.py b/amaranth/vendor/_lattice_machxo_2_3l.py
index 05f48a6..7baf605 100644
--- a/amaranth/vendor/_lattice_machxo_2_3l.py
+++ b/amaranth/vendor/_lattice_machxo_2_3l.py
@@ -101,6 +101,7 @@ class LatticeMachXO2Or3LPlatform(TemplatedPlatform):
             {{get_override("add_preferences")|default("# (add_preferences placeholder)")}}
         """,
         "{{name}}.sdc": r"""
+            set_hierarchy_separator {/}
             {% for net_signal, port_signal, frequency in platform.iter_clock_constraints() -%}
                 {% if port_signal is not none -%}
                     create_clock -name {{port_signal.name|tcl_escape}} -period {{1000000000/frequency}} [get_ports {{port_signal.name|tcl_escape}}]