diff --git a/src/ol/index.js b/src/ol/index.js
index e5e1c70048..d2924a56a8 100644
--- a/src/ol/index.js
+++ b/src/ol/index.js
@@ -11,13 +11,6 @@ import webgl from './webgl.js';
 export var DEFAULT_WMS_VERSION = '1.3.0';
 
 
-/**
- * @type {boolean} Enable integration with the Proj4js library.  Default is
- *     `true`.
- */
-export var ENABLE_PROJ4JS = true;
-
-
 /**
  * @type {boolean} Enable automatic reprojection of raster sources. Default is
  *     `true`.
@@ -143,7 +136,6 @@ export default {
   inherits: inherits,
   VERSION: VERSION,
   DEFAULT_WMS_VERSION: DEFAULT_WMS_VERSION,
-  ENABLE_PROJ4JS: ENABLE_PROJ4JS,
   ENABLE_RASTER_REPROJECTION: ENABLE_RASTER_REPROJECTION,
   DEBUG_WEBGL: DEBUG_WEBGL,
   HAS_WEBGL: HAS_WEBGL,
diff --git a/src/ol/proj.js b/src/ol/proj.js
index d28c2616d1..dd5a47e98f 100644
--- a/src/ol/proj.js
+++ b/src/ol/proj.js
@@ -1,7 +1,6 @@
 /**
  * @module ol/proj
  */
-import {ENABLE_PROJ4JS} from './index.js';
 import _ol_Sphere_ from './Sphere.js';
 import _ol_extent_ from './extent.js';
 import _ol_math_ from './math.js';
@@ -32,23 +31,21 @@ _ol_proj_.METERS_PER_UNIT = _ol_proj_Units_.METERS_PER_UNIT;
 _ol_proj_.SPHERE_ = new _ol_Sphere_(_ol_Sphere_.DEFAULT_RADIUS);
 
 
-if (ENABLE_PROJ4JS) {
-  /**
-   * Register proj4. If not explicitly registered, it will be assumed that
-   * proj4js will be loaded in the global namespace. For example in a
-   * browserify ES6 environment you could use:
-   *
-   *     import ol from 'openlayers';
-   *     import proj4 from 'proj4';
-   *     ol.proj.setProj4(proj4);
-   *
-   * @param {Proj4} proj4 Proj4.
-   * @api
-   */
-  _ol_proj_.setProj4 = function(proj4) {
-    _ol_proj_proj4_.set(proj4);
-  };
-}
+/**
+ * Register proj4. If not explicitly registered, it will be assumed that
+ * proj4js will be loaded in the global namespace. For example in a
+ * browserify ES6 environment you could use:
+ *
+ *     import ol from 'openlayers';
+ *     import proj4 from 'proj4';
+ *     ol.proj.setProj4(proj4);
+ *
+ * @param {Proj4} proj4 Proj4.
+ * @api
+ */
+_ol_proj_.setProj4 = function(proj4) {
+  _ol_proj_proj4_.set(proj4);
+};
 
 
 /**
@@ -311,7 +308,7 @@ _ol_proj_.get = function(projectionLike) {
   } else if (typeof projectionLike === 'string') {
     var code = projectionLike;
     projection = _ol_proj_projections_.get(code);
-    if (ENABLE_PROJ4JS && !projection) {
+    if (!projection) {
       var proj4js = _ol_proj_proj4_.get();
       if (typeof proj4js == 'function' &&
           proj4js.defs(code) !== undefined) {
@@ -380,7 +377,7 @@ _ol_proj_.getTransformFromProjections = function(sourceProjection, destinationPr
   var sourceCode = sourceProjection.getCode();
   var destinationCode = destinationProjection.getCode();
   var transform = _ol_proj_transforms_.get(sourceCode, destinationCode);
-  if (ENABLE_PROJ4JS && !transform) {
+  if (!transform) {
     var proj4js = _ol_proj_proj4_.get();
     if (typeof proj4js == 'function') {
       var sourceDef = proj4js.defs(sourceCode);
diff --git a/src/ol/proj/Projection.js b/src/ol/proj/Projection.js
index 6a08d1a8ac..6e925544ed 100644
--- a/src/ol/proj/Projection.js
+++ b/src/ol/proj/Projection.js
@@ -1,7 +1,6 @@
 /**
  * @module ol/proj/Projection
  */
-import {ENABLE_PROJ4JS} from '../index.js';
 import _ol_proj_Units_ from '../proj/Units.js';
 import _ol_proj_proj4_ from '../proj/proj4.js';
 
@@ -108,20 +107,18 @@ var _ol_proj_Projection_ = function(options) {
   this.metersPerUnit_ = options.metersPerUnit;
 
   var code = options.code;
-  if (ENABLE_PROJ4JS) {
-    var proj4js = _ol_proj_proj4_.get();
-    if (typeof proj4js == 'function') {
-      var def = proj4js.defs(code);
-      if (def !== undefined) {
-        if (def.axis !== undefined && options.axisOrientation === undefined) {
-          this.axisOrientation_ = def.axis;
-        }
-        if (options.metersPerUnit === undefined) {
-          this.metersPerUnit_ = def.to_meter;
-        }
-        if (options.units === undefined) {
-          this.units_ = def.units;
-        }
+  var proj4js = _ol_proj_proj4_.get();
+  if (typeof proj4js == 'function') {
+    var def = proj4js.defs(code);
+    if (def !== undefined) {
+      if (def.axis !== undefined && options.axisOrientation === undefined) {
+        this.axisOrientation_ = def.axis;
+      }
+      if (options.metersPerUnit === undefined) {
+        this.metersPerUnit_ = def.to_meter;
+      }
+      if (options.units === undefined) {
+        this.units_ = def.units;
       }
     }
   }