diff --git a/deployment/live/monitoring/ci/terragrunt.hcl b/deployment/live/monitoring/ci/terragrunt.hcl
index 94ba5aa..5d0260e 100644
--- a/deployment/live/monitoring/ci/terragrunt.hcl
+++ b/deployment/live/monitoring/ci/terragrunt.hcl
@@ -6,6 +6,7 @@ include "root" {
 inputs = merge(
   include.root.locals,
   {
+    alert_lt_num_witness_threshold = 0
   }
 )
 
diff --git a/deployment/live/monitoring/dev/terragrunt.hcl b/deployment/live/monitoring/dev/terragrunt.hcl
index 94ba5aa..5d0260e 100644
--- a/deployment/live/monitoring/dev/terragrunt.hcl
+++ b/deployment/live/monitoring/dev/terragrunt.hcl
@@ -6,6 +6,7 @@ include "root" {
 inputs = merge(
   include.root.locals,
   {
+    alert_lt_num_witness_threshold = 0
   }
 )
 
diff --git a/deployment/live/monitoring/prod/terragrunt.hcl b/deployment/live/monitoring/prod/terragrunt.hcl
index 94ba5aa..8acb465 100644
--- a/deployment/live/monitoring/prod/terragrunt.hcl
+++ b/deployment/live/monitoring/prod/terragrunt.hcl
@@ -6,6 +6,8 @@ include "root" {
 inputs = merge(
   include.root.locals,
   {
+    alert_lt_num_witness_threshold = 10
+    alert_enable_num_witness = false
   }
 )
 
diff --git a/deployment/live/monitoring/terragrunt.hcl b/deployment/live/monitoring/terragrunt.hcl
index bb08359..21a805c 100644
--- a/deployment/live/monitoring/terragrunt.hcl
+++ b/deployment/live/monitoring/terragrunt.hcl
@@ -3,9 +3,10 @@ terraform {
 }
 
 locals {
-  project_id  = "checkpoint-distributor"
-  region      = "us-central1"
-  env         = path_relative_to_include()
+  project_id               = "checkpoint-distributor"
+  region                   = "us-central1"
+  env                      = path_relative_to_include()
+  alert_enable_num_witness = true
 }
 
 remote_state {
diff --git a/deployment/modules/monitoring/main.tf b/deployment/modules/monitoring/main.tf
index c6f4da3..e56123e 100644
--- a/deployment/modules/monitoring/main.tf
+++ b/deployment/modules/monitoring/main.tf
@@ -83,6 +83,12 @@ resource "google_monitoring_dashboard" "witness_dashboard" {
                 "scale": "LINEAR"
               }
             }
+          },
+          {
+            "title": "Witness liveness alert chart",
+            "alertChart": {
+              "name": "${google_monitoring_alert_policy.witness_liveness.name}"
+            }
           }
         ]
       }
@@ -90,6 +96,33 @@ resource "google_monitoring_dashboard" "witness_dashboard" {
     EOF
 }
 
+resource "google_monitoring_alert_policy" "witness_liveness" {
+  enabled      = var.alert_enable_num_witness
+  display_name = "Number of live witnesses (${var.env})"
+  combiner     = "OR"
+  conditions {
+    display_name = "Number of live witnesses < ${var.alert_lt_num_witness_threshold}"
+    condition_monitoring_query_language {
+      # First group by both witness_id and instanceId, since the metric for
+      # each witness is reported by multiple instances. Then, since the
+      # timeseries across instances overlap, take the average. This ensures
+      # that the count for each witness is not double-counted across instances.
+      query = <<-EOT
+        fetch prometheus_target
+        | metric
+            'prometheus.googleapis.com/distributor_update_checkpoint_success/counter'
+        | filter (resource.namespace == 'distributor-service-${var.env}')
+        | align rate(1m)
+        | every 1m
+        | group_by [metric.witness_id, metric.instanceId], [row_count: row_count()]
+        | group_by [metric.witness_id], [row_count_mean: mean(row_count)]
+        | lt(${var.alert_lt_num_witness_threshold})
+      EOT
+      duration = "1800s"
+    }
+  }
+}
+
 resource "google_monitoring_alert_policy" "receiving_updates" {
   display_name = "Receiving Updates (${var.env})"
   combiner     = "OR"
diff --git a/deployment/modules/monitoring/variables.tf b/deployment/modules/monitoring/variables.tf
index 0b9a24f..2836bef 100644
--- a/deployment/modules/monitoring/variables.tf
+++ b/deployment/modules/monitoring/variables.tf
@@ -29,3 +29,12 @@ variable "env" {
   type        = string
 }
 
+variable "alert_lt_num_witness_threshold" {
+  description = "The lower bound alert threshold for the number of live witnesses, as measured by the distributor_update_checkpoint_success Prometheus metric."
+  type        = number
+}
+
+variable "alert_enable_num_witness" {
+  description = "Whether to enable alert_lt_num_witness_threshold."
+  type        = bool
+}
\ No newline at end of file