Alexander Gerald van Hoogenhouck-Tulleken is a British doctor and TV presenter. He is known for jointly presenting with his identical twin brother Chris several television programmes, including the CBBC children's series Operation Ouch!, and the Channel 4 show How to Lose Weight Well.

Van Tulleken has also individually presented many documentaries, including an episode of Horizon discussing male suicide and a BBC programme on the European migrant crisis. He can often be seen offering up his own body for science experimentation in diet show How to Lose Weight Well, as well as in BBC Horizon episodes "Sugar vs Fat" and "Is Binge Drinking Really That Bad?" As of July 2023, he was appearing regularly on the BBC One weekday programme Morning Live, providing a variety of medical advice. Van Tulleken is also a member of the Made of Stronger Stuff podcast on BBC Sounds.