diff --git a/packages/pg/lib/connection.js b/packages/pg/lib/connection.js
index 62d38fa69..ecf60534b 100644
--- a/packages/pg/lib/connection.js
+++ b/packages/pg/lib/connection.js
@@ -23,8 +23,11 @@ class Connection extends EventEmitter {
 
     this._keepAlive = config.keepAlive
     this._keepAliveInitialDelayMillis = config.keepAliveInitialDelayMillis
-    this.parsedStatements = {}
-    this.submittedNamedStatements = {}
+    // Prepared-statement caches are keyed by user-supplied statement names, so they must not
+    // inherit from Object.prototype: a statement named e.g. `constructor` would otherwise read
+    // back as already-prepared and skip its Parse message. See issue #3625.
+    this.parsedStatements = Object.create(null)
+    this.submittedNamedStatements = Object.create(null)
     this.ssl = config.ssl || false
     this.sslNegotiation = config.sslNegotiation || 'postgres'
     this._ending = false
diff --git a/packages/pg/lib/native/client.js b/packages/pg/lib/native/client.js
index 9ec3c8c03..6cbc624a8 100644
--- a/packages/pg/lib/native/client.js
+++ b/packages/pg/lib/native/client.js
@@ -57,8 +57,10 @@ const Client = (module.exports = function (config) {
   this.host = cp.host
   this.port = cp.port
 
-  // a hash to hold named queries
-  this.namedQueries = {}
+  // a hash to hold named queries; prototypeless so a query named after an
+  // Object.prototype key (e.g. `constructor`) isn't mistaken for one already
+  // prepared. See issue #3625.
+  this.namedQueries = Object.create(null)
 })
 
 Client.Query = NativeQuery
diff --git a/packages/pg/test/unit/connection/prepared-statement-name-tests.js b/packages/pg/test/unit/connection/prepared-statement-name-tests.js
new file mode 100644
index 000000000..2fdd5e529
--- /dev/null
+++ b/packages/pg/test/unit/connection/prepared-statement-name-tests.js
@@ -0,0 +1,69 @@
+'use strict'
+// Regression tests for https://github.com/brianc/node-postgres/issues/3625
+//
+// The connection tracks which prepared statements have already been sent to the
+// backend in `parsedStatements` / `submittedNamedStatements`. When those are
+// plain `{}` objects, a statement named after an `Object.prototype` key such as
+// `constructor` reads back as truthy even though it was never prepared, so the
+// client skips the `Parse` message and sends a `Bind` for a statement the
+// backend has never seen.
+const helper = require('./test-helper')
+const assert = require('assert')
+const Connection = require('../../../lib/connection')
+const Query = require('../../../lib/query')
+
+const suite = new helper.Suite()
+const { MemoryStream } = helper
+
+const PROTOTYPE_STATEMENT_NAMES = ['constructor', 'hasOwnProperty', 'toString', '__proto__']
+
+const makeConnection = function () {
+  const con = new Connection({ stream: new MemoryStream() })
+  con.connect()
+  return con
+}
+
+suite.test('a fresh connection has no prepared statements for Object.prototype names', function () {
+  const con = makeConnection()
+  for (const name of PROTOTYPE_STATEMENT_NAMES) {
+    assert.ok(
+      !con.parsedStatements[name],
+      `parsedStatements should not report '${name}' as already parsed on a fresh connection`
+    )
+    assert.ok(
+      !con.submittedNamedStatements[name],
+      `submittedNamedStatements should not report '${name}' as already submitted on a fresh connection`
+    )
+  }
+})
+
+suite.test('a statement named "constructor" is parsed before it is bound', function () {
+  const con = makeConnection()
+
+  const parsed = []
+  const bound = []
+  con.parse = function (query) {
+    parsed.push(query)
+  }
+  con.bind = function (config) {
+    bound.push(config)
+  }
+
+  const query = new Query({ text: 'SELECT $1::text', name: 'constructor', values: ['ok'] })
+  const err = query.submit(con)
+
+  assert.ifError(err)
+  assert.equal(parsed.length, 1, 'a Parse message must be sent for a never-before-seen "constructor" statement')
+  assert.equal(parsed[0].name, 'constructor')
+  assert.equal(parsed[0].text, 'SELECT $1::text')
+  assert.equal(bound.length, 1, 'the statement should also be bound after being parsed')
+  assert.equal(bound[0].statement, 'constructor')
+})
+
+suite.test('hasBeenParsed is false for an unprepared Object.prototype-named statement', function () {
+  const con = makeConnection()
+  for (const name of PROTOTYPE_STATEMENT_NAMES) {
+    const query = new Query({ text: 'SELECT 1', name })
+    assert.ok(!query.hasBeenParsed(con), `hasBeenParsed must be false for the unprepared statement named '${name}'`)
+  }
+})