The 28-year-old won in four hours three minutes nine seconds, more than eight minutes faster than Belgium's Pieter Heemeryck, with fellow Briton Mark Buckingham third.
In the women's race, Emma Pallant claimed victory from fellow Briton Lucy Charles, finishing in 4:35:15.
Switzerland's Daniela Ryf was third.
The race in the Canary Islands involved swimming 1.9km, cycling 90km and running a half-marathon (13.1 miles or 21.09km). The Ironman 70.3 event is half the regular Ironman distance.
The men's Olympic triathlon - which Brownlee won in 2012 and 2016 - involves a 1.5km swim, a 40km cycle and 10km run.
Double Olympic champion Alistair Brownlee marked his debut in long-distance triathlons with victory in an Ironman 70.3 event in Gran Canaria.