Tricia McMillan aka Trillian is a fictional character from Douglas Adams' series The Hitchhiker's Guide to the Galaxy.

Trillian is a young lady and brilliant astrophysicist whom Arthur Dent completely failed to chat up at a party at a flat (possibly above an Estate Agent's) in Islington. Arthur was fairly certain she was a young lady, but at the time was totally unaware of her academic qualifications. The trilogy later reveals that Trillian eventually left the party with an alien by the name of Zaphod Beeblebrox, who is directly responsible for her nickname. The next time she and Arthur meet is on a spaceship in deep space, a few months after the aforementioned party and shortly after the Earth has been destroyed to make way for a hyperspace bypass... which is of course highly improbable.

In the radio series she is carried off and forcibly married to the President of the Algolian Chapter of the Galactic Rotary Club, although the new series reveals this (probably) only occurred in the artificial universe within the Guide offices. In the books, which the new series follows, she saves the universe from the Krikketmen, and becomes a Sub-Etha Radio reporter under the name Trillian Astra.

The Trillian instant messaging software was named after this character.

Appears in:

• The Hitchhiker's Guide to the Galaxy
• The Restaurant at the End of the Universe
• Life, the Universe, and Everything
• Mostly Harmless

Trillian was played on radio by Susan Sheridan , on television by Sandra Dickinson , and on film by Zooey Deschanel.