diff --git a/meson.build b/meson.build
index 8ce06800b..0e74520ca 100644
--- a/meson.build
+++ b/meson.build
@@ -445,6 +445,7 @@ add_project_arguments(
   '-Werror=unused-result',
   '-Wdeprecated-copy',
   '-Wignored-qualifiers',
+  '-Werror=suggest-override',
   # Enable assertions in libstdc++ by default. Harmless on libc++. Benchmarked
   # at ~1% overhead in `nix search`.
   #
diff --git a/src/libexpr/json-to-value.cc b/src/libexpr/json-to-value.cc
index b69216a48..2309b6c97 100644
--- a/src/libexpr/json-to-value.cc
+++ b/src/libexpr/json-to-value.cc
@@ -82,28 +82,28 @@ class JSONSax : nlohmann::json_sax<json> {
 public:
     JSONSax(EvalState & state, Value & v) : state(state), rs(new JSONState(&v)) {};
 
-    bool null()
+    bool null() override
     {
         rs->value(state).mkNull();
         rs->add();
         return true;
     }
 
-    bool boolean(bool val)
+    bool boolean(bool val) override
     {
         rs->value(state).mkBool(val);
         rs->add();
         return true;
     }
 
-    bool number_integer(number_integer_t val)
+    bool number_integer(number_integer_t val) override
     {
         rs->value(state).mkInt(val);
         rs->add();
         return true;
     }
 
-    bool number_unsigned(number_unsigned_t val_)
+    bool number_unsigned(number_unsigned_t val_) override
     {
         if (val_ > std::numeric_limits<NixInt::Inner>::max()) {
             throw Error("unsigned json number %1% outside of Nix integer range", val_);
@@ -114,14 +114,14 @@ public:
         return true;
     }
 
-    bool number_float(number_float_t val, const string_t & s)
+    bool number_float(number_float_t val, const string_t & s) override
     {
         rs->value(state).mkFloat(val);
         rs->add();
         return true;
     }
 
-    bool string(string_t & val)
+    bool string(string_t & val) override
     {
         rs->value(state).mkString(val);
         rs->add();
@@ -129,7 +129,7 @@ public:
     }
 
 #if NLOHMANN_JSON_VERSION_MAJOR >= 3 && NLOHMANN_JSON_VERSION_MINOR >= 8
-    bool binary(binary_t&)
+    bool binary(binary_t&) override
     {
         // This function ought to be unreachable
         assert(false);
@@ -137,35 +137,37 @@ public:
     }
 #endif
 
-    bool start_object(std::size_t len)
+    bool start_object(std::size_t len) override
     {
         rs = std::make_unique<JSONObjectState>(std::move(rs));
         return true;
     }
 
-    bool key(string_t & name)
+    bool key(string_t & name) override
     {
         dynamic_cast<JSONObjectState*>(rs.get())->key(name, state);
         return true;
     }
 
-    bool end_object() {
+    bool end_object() override {
         rs = rs->resolve(state);
         rs->add();
         return true;
     }
 
-    bool end_array() {
+    bool end_array() override {
         return end_object();
     }
 
-    bool start_array(size_t len) {
+    bool start_array(size_t len) override {
         rs = std::make_unique<JSONListState>(std::move(rs),
             len != std::numeric_limits<size_t>::max() ? len : 128);
         return true;
     }
 
-    bool parse_error(std::size_t, const std::string&, const nlohmann::detail::exception& ex) {
+    bool
+    parse_error(std::size_t, const std::string &, const nlohmann::detail::exception & ex) override
+    {
         throw JSONParseError("%s", ex.what());
     }
 };
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 660512e49..7d1d339e8 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -976,7 +976,7 @@ bool LocalDerivationGoal::isAllowed(const DerivedPath & req)
 struct RestrictedStoreConfig : virtual LocalFSStoreConfig
 {
     using LocalFSStoreConfig::LocalFSStoreConfig;
-    const std::string name() { return "Restricted Store"; }
+    const std::string name() override { return "Restricted Store"; }
 };
 
 /* A wrapper around LocalStore that only allows building/querying of
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index 4162015d6..b09645f46 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -158,7 +158,7 @@ struct TunnelSink : Sink
 {
     Sink & to;
     TunnelSink(Sink & to) : to(to) { }
-    void operator () (std::string_view data)
+    void operator () (std::string_view data) override
     {
         to << STDERR_WRITE << data;
     }
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 9b1892bbb..6c637bd35 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -211,7 +211,7 @@ struct TeeSink : Sink
 {
     Sink & sink1, & sink2;
     TeeSink(Sink & sink1, Sink & sink2) : sink1(sink1), sink2(sink2) { }
-    virtual void operator () (std::string_view data)
+    virtual void operator () (std::string_view data) override
     {
         sink1(data);
         sink2(data);
@@ -228,7 +228,7 @@ struct TeeSource : Source
     Sink & sink;
     TeeSource(Source & orig, Sink & sink)
         : orig(orig), sink(sink) { }
-    size_t read(char * data, size_t len)
+    size_t read(char * data, size_t len) override
     {
         size_t n = orig.read(data, len);
         sink({data, n});
@@ -245,7 +245,7 @@ struct SizedSource : Source
     size_t remain;
     SizedSource(Source & orig, size_t size)
         : orig(orig), remain(size) { }
-    size_t read(char * data, size_t len)
+    size_t read(char * data, size_t len) override
     {
         if (this->remain <= 0) {
             throw EndOfFile("sized: unexpected end-of-file");
@@ -338,7 +338,7 @@ struct GeneratorSource : Source
 {
     GeneratorSource(Generator<Bytes> && g) : g(std::move(g)) {}
 
-    virtual size_t read(char * data, size_t len)
+    virtual size_t read(char * data, size_t len) override
     {
         // we explicitly do not poll the generator multiple times to fill the
         // buffer, only to produce some output at all. this is allowed by the