Alex O'Loughlin (born Alexander O'Lachlan; 24 August 1976) is an Australian actor, currently starring in CBS's Hawaii Five-0 as Lieutenant Commander Steve McGarrett. His best known earlier roles include Kevin Hiatt in The Shield, Mick St. John in Moonlight, Dr. Andy Yablonski in Three Rivers, and Stan in The Back-up Plan. He is also the current ambassador for Donate Life America.

O'Loughlin was born in Canberra, Australia. He is of Irish and Scottish descent. He was educated at Macquarie Primary School with his sister Jennifer who is a year or two younger than him. His year of birth has been given, variously, as 1975, 1976, 1977, and 1978. His father is a physics and astronomy teacher in Sydney and his mother is a nurse.

O'Loughlin suffered from asthma,attention deficit hyperactivity disorder (ADHD or ADD) and obsessive compulsive disorder (OCD). As a boy he wanted to become a jet fighter pilot when he grew up, but was told by his teacher that it was an unlikely dream as he suffers from asthma; this was a painful memory for him that recently evoked the response, "Always teach children to reach for the stars." He was a fan of heavy metal music and Monkey. Playing dress up with items out of an old trunk was a favorite pastime for young Alex. He has fond memories of often donning a white clown suit with big red polka dots. School was a continuous struggle, caused by learning difficulties that were compounded by ADHD and OCD, and he left when he was 14, after which he held all manner of jobs in a wide range of fields.