Tamsin Margaret Mary Greig (/ˈtæmzɪn ˈɡrɛɡ/; born 12 July 1966) is an English actress, known for both dramatic and comedic roles. She played Fran Katzenjammer in the Channel 4 sitcom Black Books, Dr Caroline Todd in the Channel 4 sitcom Green Wing, Beverly Lincoln in British-American sitcom Episodes and Jackie Goodman in the Channel 4 sitcom Friday Night Dinner. Other roles include Alice Chenery in BBC One's comedy-drama series Love Soup, Debbie Aldridge in BBC Radio 4's soap opera The Archers, Miss Bates in the 2009 BBC version of Jane Austen's Emma, and Beth Hardiment in the 2010 film version of Tamara Drewe. In 2020, Greig starred as Anne Trenchard in Julian Fellowes' ITV series Belgravia.

Greig is also a stage actress. She won a Laurence Olivier Award for Best Actress in 2007 for Much Ado About Nothing, and was nominated in 2011 and 2015 for her roles in The Little Dog Laughed and Women on the Verge of a Nervous Breakdown.