diff --git a/amaranth/build/plat.py b/amaranth/build/plat.py
index 3b706ab..159530f 100644
--- a/amaranth/build/plat.py
+++ b/amaranth/build/plat.py
@@ -69,13 +69,20 @@ class Platform(ResourceManager, metaclass=ABCMeta):
                 yield filename
 
     @property
-    def _deprecated_toolchain_env_var(self):
-        return f"NMIGEN_ENV_{tool_env_var(self.toolchain)}"
+    def _deprecated_toolchain_env_vars(self):
+        return (
+            f"NMIGEN_ENV_{self.toolchain}",
+            f"AMARANTH_ENV_{self.toolchain}",
+        )
 
     @property
     def _toolchain_env_var(self):
         return f"AMARANTH_ENV_{tool_env_var(self.toolchain)}"
 
+    @property
+    def _all_toolchain_env_vars(self):
+        return self._deprecated_toolchain_env_vars + (self._toolchain_env_var,)
+
     def build(self, elaboratable, name="top",
               build_dir="build", do_build=True,
               program_opts=None, do_program=False,
@@ -92,8 +99,8 @@ class Platform(ResourceManager, metaclass=ABCMeta):
         #     may fail.
         # This is OK because even if `require_tool` succeeds, the toolchain might be broken anyway.
         # The check only serves to catch common errors earlier.
-        if do_build and (self._deprecated_toolchain_env_var not in os.environ and
-                         self._toolchain_env_var not in os.environ):
+        got_env_var = any(v in os.environ for v in self._all_toolchain_env_vars)
+        if do_build and not got_env_var:
             for tool in self.required_tools:
                 require_tool(tool)
 
@@ -108,8 +115,7 @@ class Platform(ResourceManager, metaclass=ABCMeta):
         self.toolchain_program(products, name, **(program_opts or {}))
 
     def has_required_tools(self):
-        if (self._deprecated_toolchain_env_var in os.environ or
-                self._toolchain_env_var in os.environ):
+        if any(v in os.environ for v in self._all_toolchain_env_vars):
             return True
         return all(has_tool(name) for name in self.required_tools)
 
@@ -275,15 +281,17 @@ class TemplatedPlatform(Platform):
         "build_{{name}}.sh": """
             # {{autogenerated}}
             set -e{{verbose("x")}}
-            [ -n "${{platform._deprecated_toolchain_env_var}}" ] && . "${{platform._deprecated_toolchain_env_var}}"
-            [ -n "${{platform._toolchain_env_var}}" ] && . "${{platform._toolchain_env_var}}"
+            {% for var in platform._all_toolchain_env_vars %}
+            [ -n "${{var}}" ] && . "${{var}}"
+            {% endfor %}
             {{emit_commands("sh")}}
         """,
         "build_{{name}}.bat": """
             @rem {{autogenerated}}
             {{quiet("@echo off")}}
-            if defined {{platform._deprecated_toolchain_env_var}} call %{{platform._deprecated_toolchain_env_var}}%
-            if defined {{platform._toolchain_env_var}} call %{{platform._toolchain_env_var}}%
+            {% for var in platform._all_toolchain_env_vars %}
+            if defined {{var}} call %{{var}}%
+            {% endfor %}
             {{emit_commands("bat")}}
         """,
     }
diff --git a/amaranth/vendor/intel.py b/amaranth/vendor/intel.py
index e7b7c94..b1ebb5a 100644
--- a/amaranth/vendor/intel.py
+++ b/amaranth/vendor/intel.py
@@ -84,16 +84,13 @@ class IntelPlatform(TemplatedPlatform):
         **TemplatedPlatform.build_script_templates,
         "build_{{name}}.sh": r"""
             # {{autogenerated}}
-            if [ -n "${{platform._deprecated_toolchain_env_var}}" ]; then
-                QUARTUS_ROOTDIR=$(dirname $(dirname "${{platform._deprecated_toolchain_env_var}}"))
+            {% for var in platform._all_toolchain_env_vars %}
+            if [ -n "${{var}}" ]; then
+                QUARTUS_ROOTDIR=$(dirname $(dirname "${{var}}"))
                 # Quartus' qenv.sh does not work with `set -e`.
-                . "${{platform._deprecated_toolchain_env_var}}"
-            fi
-            if [ -n "${{platform._toolchain_env_var}}" ]; then
-                QUARTUS_ROOTDIR=$(dirname $(dirname "${{platform._toolchain_env_var}}"))
-                # Quartus' qenv.sh does not work with `set -e`.
-                . "${{platform._toolchain_env_var}}"
+                . "${{var}}"
             fi
+            {% endfor %}
             set -e{{verbose("x")}}
             {{emit_commands("sh")}}
         """,
diff --git a/amaranth/vendor/lattice_ecp5.py b/amaranth/vendor/lattice_ecp5.py
index 4d5cb7c..eb348ca 100644
--- a/amaranth/vendor/lattice_ecp5.py
+++ b/amaranth/vendor/lattice_ecp5.py
@@ -189,14 +189,12 @@ class LatticeECP5Platform(TemplatedPlatform):
             # {{autogenerated}}
             set -e{{verbose("x")}}
             if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi
-            if [ -n "${{platform._deprecated_toolchain_env_var}}" ]; then
-                bindir=$(dirname "${{platform._deprecated_toolchain_env_var}}")
-                . "${{platform._deprecated_toolchain_env_var}}"
-            fi
-            if [ -n "${{platform._toolchain_env_var}}" ]; then
-                bindir=$(dirname "${{platform._toolchain_env_var}}")
-                . "${{platform._toolchain_env_var}}"
+            {% for var in platform._all_toolchain_env_vars %}
+            if [ -n "${{var}}" ]; then
+                bindir=$(dirname "${{var}}")
+                . "${{var}}"
             fi
+            {% endfor %}
             {{emit_commands("sh")}}
         """,
         "{{name}}.v": r"""
diff --git a/amaranth/vendor/lattice_ice40.py b/amaranth/vendor/lattice_ice40.py
index 0e3deba..8dcada9 100644
--- a/amaranth/vendor/lattice_ice40.py
+++ b/amaranth/vendor/lattice_ice40.py
@@ -180,31 +180,22 @@ class LatticeICE40Platform(TemplatedPlatform):
         "build_{{name}}.sh": r"""
             # {{autogenerated}}
             set -e{{verbose("x")}}
-            if [ -n "${{platform._deprecated_toolchain_env_var}}" ]; then
+            {% for var in platform._all_toolchain_env_vars %}
+            if [ -n "${{var}}" ]; then
                 # LSE environment
-                export LD_LIBRARY_PATH=${{platform._deprecated_toolchain_env_var}}/LSE/bin/lin64:$LD_LIBRARY_PATH
-                export PATH=${{platform._deprecated_toolchain_env_var}}/LSE/bin/lin64:$PATH
-                export FOUNDRY=${{platform._deprecated_toolchain_env_var}}/LSE
+                export LD_LIBRARY_PATH=${{var}}/LSE/bin/lin64:$LD_LIBRARY_PATH
+                export PATH=${{var}}/LSE/bin/lin64:$PATH
+                export FOUNDRY=${{var}}/LSE
                 # Synplify environment
-                export LD_LIBRARY_PATH=${{platform._deprecated_toolchain_env_var}}/sbt_backend/bin/linux/opt/synpwrap:$LD_LIBRARY_PATH
-                export PATH=${{platform._deprecated_toolchain_env_var}}/sbt_backend/bin/linux/opt/synpwrap:$PATH
-                export SYNPLIFY_PATH=${{platform._deprecated_toolchain_env_var}}/synpbase
+                export LD_LIBRARY_PATH=${{var}}/sbt_backend/bin/linux/opt/synpwrap:$LD_LIBRARY_PATH
+                export PATH=${{var}}/sbt_backend/bin/linux/opt/synpwrap:$PATH
+                export SYNPLIFY_PATH=${{var}}/synpbase
                 # Common environment
-                export SBT_DIR=${{platform._deprecated_toolchain_env_var}}/sbt_backend
-            elif [ -n "${{platform._toolchain_env_var}}" ]; then
-                # LSE environment
-                export LD_LIBRARY_PATH=${{platform._toolchain_env_var}}/LSE/bin/lin64:$LD_LIBRARY_PATH
-                export PATH=${{platform._toolchain_env_var}}/LSE/bin/lin64:$PATH
-                export FOUNDRY=${{platform._toolchain_env_var}}/LSE
-                # Synplify environment
-                export LD_LIBRARY_PATH=${{platform._toolchain_env_var}}/sbt_backend/bin/linux/opt/synpwrap:$LD_LIBRARY_PATH
-                export PATH=${{platform._toolchain_env_var}}/sbt_backend/bin/linux/opt/synpwrap:$PATH
-                export SYNPLIFY_PATH=${{platform._toolchain_env_var}}/synpbase
-                # Common environment
-                export SBT_DIR=${{platform._toolchain_env_var}}/sbt_backend
+                export SBT_DIR=${{var}}/sbt_backend
             else
                 echo "Variable ${{platform._toolchain_env_var}} must be set" >&2; exit 1
             fi
+            {% endfor %}
             {{emit_commands("sh")}}
         """,
         "{{name}}.v": r"""
diff --git a/amaranth/vendor/lattice_machxo_2_3l.py b/amaranth/vendor/lattice_machxo_2_3l.py
index 42d9ffc..80ab54b 100644
--- a/amaranth/vendor/lattice_machxo_2_3l.py
+++ b/amaranth/vendor/lattice_machxo_2_3l.py
@@ -55,14 +55,12 @@ class LatticeMachXO2Or3LPlatform(TemplatedPlatform):
             # {{autogenerated}}
             set -e{{verbose("x")}}
             if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi
-            if [ -n "${{platform._deprecated_toolchain_env_var}}" ]; then
-                bindir=$(dirname "${{platform._deprecated_toolchain_env_var}}")
-                . "${{platform._deprecated_toolchain_env_var}}"
-            fi
-            if [ -n "${{platform._toolchain_env_var}}" ]; then
-                bindir=$(dirname "${{platform._toolchain_env_var}}")
-                . "${{platform._toolchain_env_var}}"
+            {% for var in platform._all_toolchain_env_vars %}
+            if [ -n "${{var}}" ]; then
+                bindir=$(dirname "${{var}}")
+                . "${{var}}"
             fi
+            {% endfor %}
             {{emit_commands("sh")}}
         """,
         "{{name}}.v": r"""
diff --git a/amaranth/vendor/xilinx.py b/amaranth/vendor/xilinx.py
index 217aa1b..2f2a849 100644
--- a/amaranth/vendor/xilinx.py
+++ b/amaranth/vendor/xilinx.py
@@ -126,8 +126,9 @@ class XilinxPlatform(TemplatedPlatform):
             # {{autogenerated}}
             set -e{{verbose("x")}}
             if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi
-            [ -n "${{platform._deprecated_toolchain_env_var}}" ] && . "${{platform._deprecated_toolchain_env_var}}"
-            [ -n "${{platform._toolchain_env_var}}" ] && . "${{platform._toolchain_env_var}}"
+            {% for var in platform._all_toolchain_env_vars %}
+            [ -n "${{var}}" ] && . "${{var}}"
+            {% endfor %}
             {{emit_commands("sh")}}
         """,
         "{{name}}.v": r"""
@@ -233,8 +234,9 @@ class XilinxPlatform(TemplatedPlatform):
             # {{autogenerated}}
             set -e{{verbose("x")}}
             if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi
-            [ -n "${{platform._deprecated_toolchain_env_var}}" ] && . "${{platform._deprecated_toolchain_env_var}}"
-            [ -n "${{platform._toolchain_env_var}}" ] && . "${{platform._toolchain_env_var}}"
+            {% for var in platform._all_toolchain_env_vars %}
+            [ -n "${{var}}" ] && . "${{var}}"
+            {% endfor %}
             {{emit_commands("sh")}}
         """,
         "{{name}}.v": r"""