Sean Conway used a bamboo bike and carried all his gear on the challenge, which started and finished at Lulworth, Dorset.
The 34-year-old, from Cheltenham, took 85 days to cycle, run and swim in stages around the coast.
He described the circumnavigation as "without doubt, the hardest thing that I have ever done".
Having headed west from Dorset on his bike in April, he pedalled around the coast to Scarborough where he left his bike and ran to Brighton. Once there he took to the water for the final leg back to Lulworth.
He is believed to be the first person to have completed such an "ultra-triathlon".
He was unsupported throughout the challenge and carried his own kit, including the tent that he slept in. During the swimming leg, he pulled his gear with him on a homemade raft.
Mr Conway said: "My body feels broken and I've lost all my body fat as a result, but it has all been worth it.
"The support from the public has been incredible and has kept me going. I can't thank you all enough.
"People sometimes underestimate how amazing Britain really is, and let me tell you, having circumnavigated it, this really is a beautiful island that we live on."
To consume the 5,000 calories a day he needed, he foraged for wild garlic leaves, fished for mackerel and ate dog treats and liquidised roast dinners.
A television production team tracked his progress and a documentary is due to be screened on the Discovery Channel later in the summer.
An extreme adventurer has completed a 4,000-mile ultra-triathlon around the coastline of Britain.