Mark Renton is a fictional character who appears as the protagonist in the novels Trainspotting (1993) and Porno (2002) written by Irvine Welsh.

He attempts to quit heroin, but finds it too difficult and resorts to buying opium suppositories from shady dealer Mikey Forrester.

He meets once again with Spud, Begbie and Sick Boy, as well as another friend "Second Prize", and the five sell a large amount of heroin to a dealer for £16,000.

After celebrating in a pub, Renton decides to steal the money from his friends and walk away to a new life, leaving a small share for Spud.

One night, Renton sees Carl Ewart, a DJ from Edinburgh (a character from Welsh's novel Glue), who recognises him.

Renton takes Begbie to hospital and flees Edinburgh for the last time with £60,000 which he stole from Sick Boy, and two of his friends Nikki and Diane.

Sick Boy visits Begbie in the hospital as the two plot to find and kill Renton and take back their stolen money.

In the film, Mark Renton is a 26-year-old, unemployed man who lives with his parents in Edinburgh, engaging in heroin use with Sick Boy (Jonny Lee Miller), Spud (Ewen Bremner) and their dealer Swanney "Mother Superior" (Peter Mullan).

Soon after, he is tracked down by Begbie, who is a fugitive back home after robbing a jewellery store, and Sick Boy, now a pimp and drug dealer.

They reunite with the recently released Spud, and the four buy £4,000 worth of heroin from Mikey Forrester, which they resell to a well-connected dealer in London for £16,000.

They celebrate at a pub, but Renton feels disillusioned by his friends after Sick Boy admits that he would steal the money and Begbie violently attacks an innocent man after his suit is stained by beer.

Renton and Sick Boy leave Begbie outside a prison and resume their friendship, while Veronika returns to Bulgaria and Spud reconciles with his family.

In his review for the Los Angeles Times, Kenneth Turan wrote, "in McGregor ... the film has an actor whose magnetism monopolizes our attention no matter what".