You can design a lettermark logo online using various graphic design tools and logo makers. Here are some popular online platforms where you can create your own lettermark logo:

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1. **Canva**: Canva offers a user-friendly online design tool with a wide range of templates and fonts to create lettermark logos. It’s a great choice for beginners.

2. **Looka (formerly Logojoy)**: Looka uses AI to generate logo designs based on your input. You can customize and design your lettermark logo easily.

3. **Tailor Brands**: Tailor Brands is an AI-powered logo maker that provides various design options, including lettermark logos. It’s user-friendly and offers customization features.

4. **Hatchful by Shopify**: Shopify’s Hatchful is a free online logo maker that offers templates and customization options for creating lettermark logos.

5. **Wix Logo Maker**: If you’re using Wix for your website, their integrated logo maker can help you design a lettermark logo that matches your site’s design.

6. **Looka (formerly LogoMakr)**: Looka (formerly LogoMakr) is a straightforward online logo maker with a simple interface that allows you to create your lettermark logo quickly.

7. **Adobe Spark**: Adobe Spark offers a logo maker tool with typography and design options, and it’s part of the Adobe Creative Cloud suite for more advanced customization.

8. **Designhill Logo Maker**: Designhill offers a logo maker tool that provides a range of fonts and customization options for creating lettermark logos.

These platforms offer a range of features, customization options, and pricing models, so you can choose the one that best fits your design needs and budget. Whether you’re a beginner or have some design experience, these online tools can help you create a professional lettermark logo for your brand or project.