- Ewan McGregor as Mark "Rent Boy" Renton
- Hamish Haggerty as young Renton
- Ben Skelton as 9-year-old Renton
- Connor McIndoe as 20-year-old Renton
- Ewen Bremner as Daniel "Spud" Murphy
- Aiden Haggarty as 9-year-old Spud
- John Bell as 20-year-old Spud
- Jonny Lee Miller as Simon "Sick Boy" Williamson
- Logan Gillies as 9-year-old Simon
- James McElvar as 20-year-old Simon
- Robert Carlyle as Francis "Franco" Begbie
- Daniel Jackson as young Begbie
- Daniel Smith as 9-year-old Begbie
- Christopher Mullen as 20-year-old Begbie
- Kelly Macdonald as Diane Coulston
- Shirley Henderson as Gail Houston
- James Cosmo as Mr Renton, Mark's father
- Anjela Nedyalkova as Veronika Kovach
- Steven Robertson as Stoddart, Begbie's lawyer
- Elek Kish as Dozo
- Simon Weir as Jailhoose
- Bradley Welsh as Mr Doyle
- Pauline Turner as June Begbie
- Kyle Fitzpatrick as Fergus Murphy
- Charlie Hardie as 9-year-old Fergus
- Scot Greenan as Franco Begbie Jr.
- Katie Leung as Nurse
- Michael Shaw as 20-year-old Tommy
- Elijah Wolf as 9-year-old Tommy
Archival footage included Eileen Nicholas as Mrs Renton and Kevin McKidd as Tommy MacKenzie from the first film. McGregor, Bremner, Miller and Carlyle also appear in footage from the original Trainspotting film. The author of Trainspotting, Irvine Welsh, appears near the middle of the film as Mikey Forrester, reprising his role from the first film. Carlyle also plays Begbie's father, the briefly seen wino in the Leith Railway Station.
