Trillian is a proprietary multiprotocol instant messaging application created by Cerulean Studios. It is currently available for Microsoft Windows, macOS, Linux, Android, iOS, BlackBerry OS, and the Web. It can connect to multiple IM services, such as AIM, Bonjour, Facebook Messenger, Google Talk (Hangouts), IRC, XMPP (Jabber), VZ, and Yahoo! Messenger networks; as well as social networking sites, such as Facebook, Foursquare, LinkedIn, and Twitter; and email services, such as POP3 and IMAP.

Trillian no longer supports Windows Live Messenger or Skype as these services have combined and Microsoft chose to discontinue Skypekit, which was used for connection. They also no longer support connecting to MySpace, or a distinct connection for Gmail, Hotmail or Yahoo! Mail although these can still be connected to via POP3 or IMAP. Currently, Trillian supports Jabber (XMPP) and Olark.

Initially released July 1, 2000, as a freeware IRC client, the first commercial version (Trillian Pro 1.0) was published on September 10, 2002. The program was named after Trillian, a fictional character in The Hitchhiker's Guide to the Galaxy by Douglas Adams. A previous version of the official web site even had a tribute to Douglas Adams on its front page. On August 14, 2009, Trillian "Astra" (4.0) for Windows was released, along with its own Astra network. Trillian 5 for Windows was released in May 2011, and Trillian 6.0 was initially released in February 2017.

Trillian connects to multiple instant messaging services without the need of running multiple clients. Users can create multiple connections to the same service, and can also group connections under separate identities to prevent confusion. All contacts are gathered under the same contact list. Contacts are not bound to their own IM service groups, and can be dragged and dropped freely. Trillian represents each service with a different-colored sphere. Prior versions used the corporate logos for each service, but these were removed to avoid copyright issues, although some skins still use the original icons. The Trillian designers chose a color-coding scheme based on the underground maps used by the London Underground that uses different colors to differentiate between different lines.

Prior versions of Trillian supported:

To eliminate duplicates and simplify the structure of the contact list, users can bundle multiple contact entries for the same person into one entry in the contact list, using the Metacontact feature (similarly to Ayttm's fallback messaging feature). Subcontacts will appear under the metacontact as small icons aligned in the form of a tree.

Trillian Pro comes with Activity History, and both log the history as both plain text files and as XML files. Pro has a History Manager that shows the chat history and allows the user to add bookmarks for revision later on. XML-based history makes the log easy to manipulate, searchable and extendable for future functions.

Trillian Pro also has a stream manipulation feature labelled 'time travel', which allows the user to record, and subsequently review, pause, rewind, and fast forward live video and audio sessions.