diff --git a/packages/debug/src/browser/preferences/launch-preferences.spec.ts b/packages/debug/src/browser/preferences/launch-preferences.spec.ts
index 86e82821f52e9..aef85e3b980c1 100644
--- a/packages/debug/src/browser/preferences/launch-preferences.spec.ts
+++ b/packages/debug/src/browser/preferences/launch-preferences.spec.ts
@@ -46,6 +46,9 @@ import { bindUserStorage } from '@theia/userstorage/lib/browser/user-storage-fro
 import { FileSystemWatcherServer } from '@theia/filesystem/lib/common/filesystem-watcher-protocol';
 import { MockFilesystemWatcherServer } from '@theia/filesystem/lib/common/test/mock-filesystem-watcher-server';
 import { bindLaunchPreferences } from './launch-preferences';
+import { EditorManager } from '@theia/editor/lib/browser/editor-manager';
+import URI from '@theia/core/lib/common/uri';
+import { EditorWidget } from '@theia/editor/lib/browser/editor-widget';
 
 disableJSDOM();
 
@@ -333,12 +336,12 @@ describe('Launch Preferences', () => {
     function testLaunchAndSettingsSuite({
         name, expectation, launch, only, configMode
     }: {
-            name: string,
-            expectation: any,
-            launch?: any,
-            only?: boolean,
-            configMode?: ConfigMode
-        }): void {
+        name: string,
+        expectation: any,
+        launch?: any,
+        only?: boolean,
+        configMode?: ConfigMode
+    }): void {
         testSuite({
             name: name + ' Launch Configuration',
             launch,
@@ -398,13 +401,13 @@ describe('Launch Preferences', () => {
     function testConfigSuite({
         configMode, expectation, inspectExpectation, settings, launch, only
     }: {
-            configMode: ConfigMode
-            expectation: any,
-            inspectExpectation?: any,
-            launch?: any,
-            settings?: any,
-            only?: boolean
-        }): void {
+        configMode: ConfigMode
+        expectation: any,
+        inspectExpectation?: any,
+        launch?: any,
+        settings?: any,
+        only?: boolean
+    }): void {
 
         describe(JSON.stringify(configMode, undefined, 2), () => {
 
@@ -458,6 +461,11 @@ describe('Launch Preferences', () => {
                 bindPreferenceProviders(bind, unbind);
                 bindWorkspacePreferences(bind);
                 container.bind(WorkspaceService).toSelf().inSingletonScope();
+                container.bind(EditorManager).toConstantValue(<EditorManager>{
+                    getByUri(uri: URI): Promise<EditorWidget | undefined> {
+                        return Promise.resolve(undefined);
+                    }
+                });
                 container.bind(WindowService).toConstantValue(new MockWindowService());
 
                 const workspaceServer = new MockWorkspaceServer();
diff --git a/packages/preferences/src/browser/abstract-resource-preference-provider.ts b/packages/preferences/src/browser/abstract-resource-preference-provider.ts
index d73c5ea13dc79..2fb665c2984ef 100644
--- a/packages/preferences/src/browser/abstract-resource-preference-provider.ts
+++ b/packages/preferences/src/browser/abstract-resource-preference-provider.ts
@@ -23,6 +23,7 @@ import { MessageService, Resource, ResourceProvider, Disposable } from '@theia/c
 import { PreferenceProvider, PreferenceSchemaProvider, PreferenceScope, PreferenceProviderDataChange } from '@theia/core/lib/browser';
 import URI from '@theia/core/lib/common/uri';
 import { PreferenceConfigurations } from '@theia/core/lib/browser/preferences/preference-configurations';
+import { EditorManager, TextEditorDocument } from '@theia/editor/lib/browser';
 
 @injectable()
 export abstract class AbstractResourcePreferenceProvider extends PreferenceProvider {
@@ -30,6 +31,7 @@ export abstract class AbstractResourcePreferenceProvider extends PreferenceProvi
     protected preferences: { [key: string]: any } = {};
     protected resource: Promise<Resource>;
 
+    @inject(EditorManager) protected readonly editorManager: EditorManager;
     @inject(ResourceProvider) protected readonly resourceProvider: ResourceProvider;
     @inject(MessageService) protected readonly messageService: MessageService;
     @inject(PreferenceSchemaProvider) protected readonly schemaProvider: PreferenceSchemaProvider;
@@ -102,9 +104,15 @@ export abstract class AbstractResourcePreferenceProvider extends PreferenceProvi
             return true;
         }
         try {
+            const editor = await this.editorManager.getByUri(new URI(resourceUri!));
+            let tabSize: number = 0;
+            let insertSpaces: boolean = true;
+            if (editor) {
+                [tabSize, insertSpaces] = this.detectIndentation(editor.editor.document);
+            }
             let newContent = '';
             if (path.length || value !== undefined) {
-                const formattingOptions = { tabSize: 3, insertSpaces: true, eol: '' };
+                const formattingOptions = { tabSize, insertSpaces, eol: '' };
                 const edits = jsoncparser.modify(content, path, value, { formattingOptions });
                 newContent = jsoncparser.applyEdits(content, edits);
             }
@@ -221,4 +229,33 @@ export abstract class AbstractResourcePreferenceProvider extends PreferenceProvi
         }
     }
 
+    /**
+     * Detect the minimum indentation and type of indentation to be used
+     * as formatting options when using `jsoncparser`.
+     * @param text the preference text editor document.
+     *
+     * @returns a tuple representing the indentation size, and if it spaces.
+     */
+    protected detectIndentation(text: TextEditorDocument): [number, boolean] {
+        // Get the number of lines present in the text document.
+        const lineCount = text.lineCount;
+        // Store the current index which will be used iterate over the document.
+        let index = 0;
+        // Iterate over the each line in the text document, getting the first indentation that is not zero.
+        while (index < lineCount) {
+            // Get the line content of the document a the given index.
+            const line = text.getLineContent(index + 1);
+            // Determine how many leading spaces are present in the document.
+            const indentation = line.search(/\S|$/);
+            // Return the first non-zero indentation.
+            if (indentation > 0) {
+                // Determine if the line is indented with spaces or tabs.
+                const isSpaces = !line.startsWith('\t');
+                return [indentation, isSpaces];
+            }
+            index++;
+        }
+        return [0, true];
+    }
+
 }