diff --git a/README.md b/README.md
index ca3844d..0058fd3 100644
--- a/README.md
+++ b/README.md
@@ -28,6 +28,7 @@ widely-used reputation systems.
 | AlienVault OTX        | HTTPS lookup    | free (admin)   | admin |
 | Pulsedive             | HTTPS lookup    | free (admin)   | admin |
 | Criminal IP           | HTTPS lookup    | yes (admin)    | admin |
+| Quad9 secure DNS      | DNS comparison  | no             | user (default on) |
 
 ### Obtaining API keys
 
diff --git a/checker/quad9.go b/checker/quad9.go
new file mode 100644
index 0000000..7bf6094
--- /dev/null
+++ b/checker/quad9.go
@@ -0,0 +1,127 @@
+package checker
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"sync"
+	"time"
+
+	sdk "git.happydns.org/checker-sdk-go/checker"
+)
+
+func init() {
+	Register(&quad9Source{
+		secure:    newDNSResolver("9.9.9.9:53"),
+		unsecured: newDNSResolver("9.9.9.10:53"),
+	})
+}
+
+type quad9Source struct {
+	secure    *net.Resolver
+	unsecured *net.Resolver
+}
+
+func newDNSResolver(addr string) *net.Resolver {
+	return &net.Resolver{
+		PreferGo: true,
+		Dial: func(ctx context.Context, network, _ string) (net.Conn, error) {
+			d := &net.Dialer{Timeout: 5 * time.Second}
+			return d.DialContext(ctx, "udp", addr)
+		},
+	}
+}
+
+func (*quad9Source) ID() string   { return "quad9" }
+func (*quad9Source) Name() string { return "Quad9 secure DNS" }
+
+func (*quad9Source) Options() SourceOptions {
+	return SourceOptions{
+		User: []sdk.CheckerOptionField{
+			{
+				Id:          "enable_quad9",
+				Type:        "bool",
+				Label:       "Use Quad9 secure DNS check",
+				Description: "Compare Quad9's secure resolver (9.9.9.9) against its unsecured peer (9.9.9.10). A domain that resolves on the unsecured but returns NXDOMAIN on the secure resolver is blocked by Quad9's threat intelligence.",
+				Default:     true,
+			},
+		},
+	}
+}
+
+func (s *quad9Source) Query(ctx context.Context, domain, registered string, opts sdk.CheckerOptions) []SourceResult {
+	if !sdk.GetBoolOption(opts, "enable_quad9", true) || registered == "" {
+		return disabledResult(s.ID(), s.Name())
+	}
+
+	res := SourceResult{
+		SourceID:   s.ID(),
+		SourceName: s.Name(),
+		Enabled:    true,
+		LookupURL:  "https://www.quad9.net/result/?domain=" + registered,
+	}
+
+	var (
+		secureAddrs, unsecuredAddrs []net.IPAddr
+		secureErr, unsecuredErr     error
+		wg                          sync.WaitGroup
+	)
+	queryCtx, cancel := context.WithTimeout(ctx, 10*time.Second)
+	defer cancel()
+
+	wg.Add(2)
+	go func() {
+		defer wg.Done()
+		secureAddrs, secureErr = s.secure.LookupIPAddr(queryCtx, registered)
+	}()
+	go func() {
+		defer wg.Done()
+		unsecuredAddrs, unsecuredErr = s.unsecured.LookupIPAddr(queryCtx, registered)
+	}()
+	wg.Wait()
+
+	// Suppress the unused variable warning — secureAddrs is only used as an
+	// existence check; the actual IPs are not surfaced.
+	_ = secureAddrs
+
+	// If the unsecured resolver can't resolve the domain either, it simply
+	// doesn't exist — not a Quad9 block.
+	if unsecuredErr != nil || len(unsecuredAddrs) == 0 {
+		if unsecuredErr != nil {
+			if dnsErr, ok := unsecuredErr.(*net.DNSError); !ok || !dnsErr.IsNotFound {
+				res.Error = "unsecured resolver: " + unsecuredErr.Error()
+			}
+		}
+		return []SourceResult{res}
+	}
+
+	// Domain resolves on unsecured. Check whether the secure resolver blocked it.
+	if dnsErr, ok := secureErr.(*net.DNSError); ok && dnsErr.IsNotFound {
+		res.Reasons = []string{"Blocked by Quad9 threat intelligence"}
+		res.Evidence = append(res.Evidence, Evidence{
+			Label: "Secure resolver (9.9.9.9)",
+			Value: "NXDOMAIN",
+		})
+	} else if secureErr != nil {
+		res.Error = "secure resolver: " + secureErr.Error()
+	}
+
+	return []SourceResult{res}
+}
+
+func (*quad9Source) Evaluate(r SourceResult) (bool, string) {
+	return evidenceEval(r, SeverityCrit)
+}
+
+func (*quad9Source) Diagnose(res SourceResult) Diagnosis {
+	return Diagnosis{
+		Severity: SeverityCrit,
+		Title:    "Blocked by Quad9 secure DNS resolver",
+		Detail: fmt.Sprintf(
+			"This domain is on Quad9's threat intelligence blocklist: the secure resolver (9.9.9.9) returns NXDOMAIN while the unsecured peer (9.9.9.10) resolves normally. Quad9 aggregates feeds from 18+ threat intel partners (abuse.ch, Bambenek, CINS, DShield, etc.). Visitors whose ISP or device uses Quad9 cannot reach this domain. Submit a false-positive report at the Quad9 contact page if you believe the listing is incorrect.",
+		),
+		Fix:       "https://www.quad9.net/support/contact/",
+		FixIsURL:  true,
+		LookupURL: res.LookupURL,
+	}
+}
diff --git a/checker/source_test.go b/checker/source_test.go
index 8f432b8..c98c8b8 100644
--- a/checker/source_test.go
+++ b/checker/source_test.go
@@ -24,7 +24,7 @@ func TestRegisteredSourcesAreSane(t *testing.T) {
 		}
 	}
 	// At least the built-in sources are present.
-	for _, want := range []string{"dnsbl", "google_safe_browsing", "openphish", "pulsedive", "urlhaus", "virustotal"} {
+	for _, want := range []string{"dnsbl", "google_safe_browsing", "openphish", "pulsedive", "quad9", "urlhaus", "virustotal"} {
 		if !seen[want] {
 			t.Errorf("missing built-in source %q", want)
 		}