diff --git a/lib/ast.js b/lib/ast.js
index c0a50d437..07f2b5010 100644
--- a/lib/ast.js
+++ b/lib/ast.js
@@ -889,12 +889,13 @@ var AST_NameMapping = DEFNODE("NameMapping", "foreign_name name", {
     },
 });
 
-var AST_Import = DEFNODE("Import", "imported_name imported_names module_name", {
+var AST_Import = DEFNODE("Import", "imported_name imported_names module_name assert_clause", {
     $documentation: "An `import` statement",
     $propdoc: {
         imported_name: "[AST_SymbolImport] The name of the variable holding the module's default export.",
         imported_names: "[AST_NameMapping*] The names of non-default imported variables",
         module_name: "[AST_String] String literal describing where this module came from",
+        assert_clause: "[AST_Object?] The import assertion"
     },
     _walk: function(visitor) {
         return visitor._visit(this, function() {
@@ -923,14 +924,15 @@ var AST_ImportMeta = DEFNODE("ImportMeta", null, {
     $documentation: "A reference to import.meta",
 });
 
-var AST_Export = DEFNODE("Export", "exported_definition exported_value is_default exported_names module_name", {
+var AST_Export = DEFNODE("Export", "exported_definition exported_value is_default exported_names module_name assert_clause", {
     $documentation: "An `export` statement",
     $propdoc: {
         exported_definition: "[AST_Defun|AST_Definitions|AST_DefClass?] An exported definition",
         exported_value: "[AST_Node?] An exported value",
         exported_names: "[AST_NameMapping*?] List of exported names",
         module_name: "[AST_String?] Name of the file to load exports from",
-        is_default: "[Boolean] Whether this is the default exported value of this module"
+        is_default: "[Boolean] Whether this is the default exported value of this module",
+        assert_clause: "[AST_Object?] The import assertion"
     },
     _walk: function (visitor) {
         return visitor._visit(this, function () {
diff --git a/lib/mozilla-ast.js b/lib/mozilla-ast.js
index fb75cca71..b17510e88 100644
--- a/lib/mozilla-ast.js
+++ b/lib/mozilla-ast.js
@@ -158,6 +158,7 @@ import {
     AST_With,
     AST_Yield,
 } from "./ast.js";
+import { is_basic_identifier_string } from "./parse.js";
 
 (function() {
 
@@ -179,6 +180,24 @@ import {
         return body;
     };
 
+    const assert_clause_from_moz = (assertions) => {
+        if (assertions && assertions.length > 0) {
+            return new AST_Object({
+                start: my_start_token(assertions),
+                end: my_end_token(assertions),
+                properties: assertions.map((assertion_kv) =>
+                    new AST_ObjectKeyVal({
+                        start: my_start_token(assertion_kv),
+                        end: my_end_token(assertion_kv),
+                        key: assertion_kv.key.name || assertion_kv.key.value,
+                        value: from_moz(assertion_kv.value)
+                    })
+                )
+            })
+        }
+        return null;
+    }
+
     var MOZ_TO_ME = {
         Program: function(M) {
             return new AST_Toplevel({
@@ -499,7 +518,8 @@ import {
                 end         : my_end_token(M),
                 imported_name: imported_name,
                 imported_names : imported_names,
-                module_name : from_moz(M.source)
+                module_name : from_moz(M.source),
+                assert_clause: assert_clause_from_moz(M.assertions)
             });
         },
         ExportAllDeclaration: function(M) {
@@ -512,7 +532,8 @@ import {
                         foreign_name: new AST_SymbolExportForeign({ name: "*" })
                     })
                 ],
-                module_name: from_moz(M.source)
+                module_name: from_moz(M.source),
+                assert_clause: assert_clause_from_moz(M.assertions)
             });
         },
         ExportNamedDeclaration: function(M) {
@@ -526,7 +547,8 @@ import {
                         name: from_moz(specifier.local)
                     });
                 }) : null,
-                module_name: from_moz(M.source)
+                module_name: from_moz(M.source),
+                assert_clause: assert_clause_from_moz(M.assertions)
             });
         },
         ExportDefaultDeclaration: function(M) {
@@ -818,12 +840,30 @@ import {
         };
     });
 
+    const assert_clause_to_moz = assert_clause => {
+        const assertions = [];
+        if (assert_clause) {
+            for (const { key, value } of assert_clause.properties) {
+                const key_moz = is_basic_identifier_string(key)
+                    ? { type: 'Identifier', name: key }
+                    : { type: 'Literal', value: key, raw: JSON.stringify(key) };
+                assertions.push({
+                    type: 'ImportAttribute',
+                    key: key_moz,
+                    value: to_moz(value)
+                });
+            }
+        }
+        return assertions;
+    }
+
     def_to_moz(AST_Export, function To_Moz_ExportDeclaration(M) {
         if (M.exported_names) {
             if (M.exported_names[0].name.name === "*") {
                 return {
                     type: "ExportAllDeclaration",
-                    source: to_moz(M.module_name)
+                    source: to_moz(M.module_name),
+                    assertions: assert_clause_to_moz(M.assert_clause)
                 };
             }
             return {
@@ -836,7 +876,8 @@ import {
                     };
                 }),
                 declaration: to_moz(M.exported_definition),
-                source: to_moz(M.module_name)
+                source: to_moz(M.module_name),
+                assertions: assert_clause_to_moz(M.assert_clause)
             };
         }
         return {
@@ -870,7 +911,8 @@ import {
         return {
             type: "ImportDeclaration",
             specifiers: specifiers,
-            source: to_moz(M.module_name)
+            source: to_moz(M.module_name),
+            assertions: assert_clause_to_moz(M.assert_clause)
         };
     });
 
diff --git a/lib/output.js b/lib/output.js
index c85b61b46..1ee6b8e52 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -1637,6 +1637,10 @@ function OutputStream(options) {
             output.space();
         }
         self.module_name.print(output);
+        if (self.assert_clause) {
+            output.print("assert");
+            self.assert_clause.print(output);
+        }
         output.semicolon();
     });
     DEFPRINT(AST_ImportMeta, function(self, output) {
@@ -1702,6 +1706,10 @@ function OutputStream(options) {
             output.space();
             self.module_name.print(output);
         }
+        if (self.assert_clause) {
+            output.print("assert");
+            self.assert_clause.print(output);
+        }
         if (self.exported_value
                 && !(self.exported_value instanceof AST_Defun ||
                     self.exported_value instanceof AST_Function ||
diff --git a/lib/parse.js b/lib/parse.js
index 3e0cd8b13..1f4ef4e80 100644
--- a/lib/parse.js
+++ b/lib/parse.js
@@ -1222,7 +1222,7 @@ function parse($TEXT, options) {
             }
             if (S.token.value == "import" && !is_token(peek(), "punc", "(") && !is_token(peek(), "punc", ".")) {
                 next();
-                var node = import_();
+                var node = import_statement();
                 semicolon();
                 return node;
             }
@@ -1374,7 +1374,7 @@ function parse($TEXT, options) {
               case "export":
                 if (!is_token(peek(), "punc", "(")) {
                     next();
-                    var node = export_();
+                    var node = export_statement();
                     if (is("punc", ";")) semicolon();
                     return node;
                 }
@@ -2666,7 +2666,15 @@ function parse($TEXT, options) {
         }
     }
 
-    function import_() {
+    function maybe_import_assertion() {
+        if (is("name", "assert") && !has_newline_before(S.token)) {
+            next();
+            return object_or_destructuring_();
+        }
+        return null
+    }
+
+    function import_statement() {
         var start = prev();
 
         var imported_name;
@@ -2689,16 +2697,20 @@ function parse($TEXT, options) {
             unexpected();
         }
         next();
+
+        const assert_clause = maybe_import_assertion();
+
         return new AST_Import({
-            start: start,
-            imported_name: imported_name,
-            imported_names: imported_names,
+            start,
+            imported_name,
+            imported_names,
             module_name: new AST_String({
                 start: mod_str,
                 value: mod_str.value,
                 quote: mod_str.quote,
                 end: mod_str,
             }),
+            assert_clause,
             end: S.token,
         });
     }
@@ -2806,7 +2818,7 @@ function parse($TEXT, options) {
         return names;
     }
 
-    function export_() {
+    function export_statement() {
         var start = S.token;
         var is_default;
         var exported_names;
@@ -2824,6 +2836,8 @@ function parse($TEXT, options) {
                 }
                 next();
 
+                const assert_clause = maybe_import_assertion();
+
                 return new AST_Export({
                     start: start,
                     is_default: is_default,
@@ -2835,6 +2849,7 @@ function parse($TEXT, options) {
                         end: mod_str,
                     }),
                     end: prev(),
+                    assert_clause
                 });
             } else {
                 return new AST_Export({
@@ -2880,6 +2895,7 @@ function parse($TEXT, options) {
             exported_value: exported_value,
             exported_definition: exported_definition,
             end: prev(),
+            assert_clause: null
         });
     }
 
diff --git a/test/compress/harmony.js b/test/compress/harmony.js
index 2276cdbc7..31c28a818 100644
--- a/test/compress/harmony.js
+++ b/test/compress/harmony.js
@@ -303,6 +303,34 @@ export_from_statement: {
     expect_exact: 'export*from"a.js";export{A}from"a.js";export{A,B}from"a.js";export{C};'
 }
 
+import_assertions: {
+    input: {
+        import "hello" assert { key: 'value' };
+    }
+    expect_exact: 'import"hello"assert{key:"value"};'
+}
+
+import_assertions_with_spaces_in_obj: {
+    input: {
+        import "hello" assert { 'k e y': 'value' };
+    }
+    expect_exact: 'import"hello"assert{"k e y":"value"};'
+}
+
+export_from_assertions: {
+    input: {
+        export * from "hello" assert { key: 'value' };
+    }
+    expect_exact: 'export*from"hello"assert{key:"value"};'
+}
+
+export_named_from_assertions: {
+    input: {
+        export { x } from "hello" assert { key: 'value' };
+    }
+    expect_exact: 'export{x}from"hello"assert{key:"value"};'
+}
+
 import_statement_mangling: {
     mangle = { toplevel: true };
     input: {