Which online services specialize in topic-based forums?

If by topic-based forums you mean online services where discussions are organized around particular interests, subjects, or communities, the main examples are:

• Reddit — Large network of topic-specific communities (“subreddits”), covering everything from hobbies and gaming to science and local interests.Reddit Help+1
• Stack Exchange — Specialized Q&A communities for subjects such as programming, mathematics, cooking, science, and philosophy. It is more question-and-answer oriented than a traditional forum.Stack Exchange+1
• Quora — Organizes questions and answers around topics that users can follow, making it useful for broad knowledge and advice discussions.Quora Help Center+2
• Discourse — Primarily forum software used to create dedicated communities. Discussions can be organized by categories and tags, with search and moderation features.Discourse - Civilized Discussion+2

In short: Reddit is oriented toward joining existing topic communities; Stack Exchange toward structured expert Q&A; Quora toward general topic-based questions and answers; and Discourse toward creating or running a dedicated forum.

When looking at online services that specialize in topic-based forums and communities , options generally split into two categories: ready-made consumer platforms (where you join existing niche discussions) and hosted software/SaaS platforms (services you use to build your own topic-based forum).

Consumer Platforms (To Join & Discuss Topics)

• Reddit: The largest macro-forum network on the web, structured entirely around micro-communities called subreddits dedicated to specific hobbies, news, regions, and niche interests.
• Discord: Originally built for gaming, it has evolved into a massive hub for real-time, topic-based text and voice communities organized into specialized servers and channels.
• Stack Exchange: A massive network of distinct, highly moderated Q&A forum communities focusing on technical and academic topics like programming, mathematics, and science.

Dedicated Forum & Community Hosting Services (To Build a Forum)

• Discourse: A modern, fully open-source and hosted discussion platform built for long-form, searchable, and structured topic categories. It is widely considered the gold standard for contemporary web forums.
• Higher Logic Vanilla: Formerly Vanilla Forums, this service specializes in sleek, highly customizable community forums with integrated knowledge bases and gamification designed to scale niche groups.
• Bettermode: A flexible customer community platform that lets brands organize topic-based discussions, Q&As, and resource hubs into clean, modular spaces.
• Mighty Networks: An all-in-one platform heavily praised for combining structured topic forums, courses, and native livestreaming into a branded community experience.
• XenForo: A powerful, premium PHP/cloud-based forum software utilized extensively by large hobbyist, gaming, and tech-support communities.

Are you looking to participate in existing communities around a specific hobby or subject, or are you searching for a platform to build and host your own topic-based forum?

If you mean online services where discussions are organized around specific topics or interests, the main examples are:

• Reddit — Large network of topic-specific communities (“subreddits”), covering everything from hobbies and technology to niche interests.Forumera
• Stack Exchange — Specialized Q&A communities for subjects such as programming, mathematics, science, cooking, and more. It is more structured than a conventional forum.Wikipedia
• Quora — Topic-oriented Q&A, particularly useful for general knowledge and explanatory questions.Forumera
• Lemmy — A decentralized, Reddit-like network where independently operated servers host topic-based communities.selfhosting.sh
• Hacker News — A more specialized discussion community focused on technology, startups, and programming.Forumera
• Discourse — Primarily software for creating and hosting dedicated discussion forums, rather than one centralized collection of communities. It supports categories, tags, threaded discussions, and moderation.Discourse - Civilized Discussion
• Discord — Primarily chat, but servers often function as topic-based communities with separate channels for different subjects.Forumera

If you're looking for places to join existing topic forums, Reddit and Stack Exchange are the clearest examples. If you're looking to create your own topic-based forum, Discourse is a more relevant category.

If you mean services where discussions are organized around particular subjects, interests, or niches, the main ones are:

• Reddit — Large network of topic-specific communities (“subreddits”), covering almost any interest. Best suited to open-ended discussion and community interaction.Parse
• Stack Exchange — A network of highly specialized Q&A communities for subjects such as programming, mathematics, science, cooking, and more. It's more structured and answer-focused than a conventional forum.Meta Stack Exchange+1
• Quora — Organizes questions and answers around broad and specialized topics. It tends toward explanatory Q&A rather than traditional threaded forums.Parse
• Discord — Primarily chat, but servers frequently function as topic-specific communities with channels devoted to particular subjects.asking.space
• Hacker News — A more specialized discussion community focused on technology, programming, startups, and related subjects.Parse
• Lemmy — A decentralized, Reddit-like service where independently operated servers host topic-based communities.Parse
• Discourse — Different from the others: it's primarily software for creating and hosting dedicated topic-based forums rather than one giant public forum.Parse

At a glance

If by “topic-based forums” you mean services specifically designed to help you discover existing forums about a particular topic, rather than platforms that host the forums themselves, I can give you a separate list of those.

If you mean online platforms where discussions are organized around particular subjects or interests, the main examples are:

• Reddit — Topic-specific communities called subreddits cover an enormous range of interests, hobbies, news, and niche subjects.
• Stack Exchange — Specialized communities for subjects such as programming, mathematics, science, cooking, and gaming. It is more structured and Q&A-focused than a traditional forum.Stack Exchange
• Quora — Questions and answers are organized around topics that users can follow.Quora Help Center+1
• Discourse — Primarily a platform for organizations and communities to create their own topic-based discussion forums, rather than one centralized forum.Discourse - Civilized Discussion
• Lemmy — A decentralized, Reddit-like network where independently operated servers host subject-specific communities.
• Discord — Primarily a chat service, but many servers function as topic-based communities with separate channels for different subjects.

In short: for joining existing topic communities, Reddit and Stack Exchange are particularly prominent; for creating and hosting your own topic-based forum, Discourse is a major option.

When looking for online services that specialize in topic-based forums and structured discussions, platforms generally fall into two categories: consumer-facing community networks (where you join existing niche discussions) and hosted forum software/platforms (where you build or host a topic-based community).

Consumer-Facing Topic-Based Communities

1. • Reddit – The massive aggregate of user-created communities (subreddits) dedicated to specific hobbies, news, regions, and niche interests. It remains the gold standard for searchable, highly-categorized public discussion threads.
2. • Stack Exchange / Stack Overflow – A heavily structured, Q&A-style network of topic-specific communities ranging from programming and mathematics to DIY and literature, built around strict peer-reviewed answers.it-maniacs.com+1
3. • Quora – A knowledge-market and discussion hybrid where conversations and informational threads are explicitly cataloged by specific subjects and topics.it-maniacs.com+1

Dedicated Forum & Community Software Providers

If you are looking to launch or participate in deeply organized, long-form, searchable topic boards rather than fast-paced chat:
4. * Discourse – Modern, open-source, and widely adopted forum software built for long-form, highly searchable, and structured topic categories rather than momentary chat.
5. * XenForo – A premium, highly polished community forum platform traditional web communities, enthusiast groups, and gaming boards rely on for deep permission controls and user-group structures.
6. * phpBB – One of the oldest and most reliable free, open-source bulletin board systems utilized by hobbyist and gaming groups.

All-in-One Creator & Business Community Platforms

1. • Mighty Networks – An expansive platform combining structured topic feeds, articles, courses, and live events geared toward creators and brands.
2. • Bettermode – (Formerly Tribe) An engagement platform tailored for businesses to embed clean, modern, topic-based feedback and discussion forums directly into their ecosystem.

Are you looking to participate in existing discussions on a specific subject, or do you need a platform to build your own topic-based forum?

Online services that specialize in topic-based forums generally fall into two groups: places where you join existing communities, and platforms used to create your own forums.Parse

Public topic-based forum communities

• reddit.com — A large network of topic-focused communities called “subreddits,” covering hobbies, technology, news, entertainment, professional interests, and niche subjects.Parse
• stackexchange.com — A network of specialized Q&A communities focused on specific subjects such as programming, mathematics, science, and other fields. It is more structured around questions and answers than open-ended discussion.Wikipedia
• quora.com — A topic-organized question-and-answer platform where users follow subjects and contribute answers and discussions.Parse
• news.ycombinator.com — A specialized discussion community focused mainly on technology, startups, programming, and related topics.Parse
• discord.com — Primarily a chat platform, but many communities organize around topics with dedicated channels and discussion areas.Parse

Services for creating your own topic-based forum

• discourse.org — A forum platform designed for building dedicated communities with categories, tags, threaded discussions, moderation tools, and searchable archives.Discourse - Civilized Discussion
• nodebb.org — An open-source forum platform for creating customizable discussion communities.Brndle
• proboards.com — A hosted service for creating traditional discussion boards and forums.Parse

A quick guide:

• Find communities on almost any topic: Reddit
• Get expert answers: Stack Exchange
• Ask broad knowledge questions: Quora
• Build a dedicated forum: Discourse
• Run a real-time community: DiscordParse