Mike Boag had dropped his ticket near London's Farringdon station and hours later set off from his home in Hackney for the Olympic stadium only to discover he did not have it, a newspaper reported.
Mr Boag, a risk consultant, searched his house and finally turned to his computer and typed "lost Olympic ticket" into a search engine.
The first result that came up was a link to the tweet: "So has anyone in the Farringdon area lost an Olympic Games ticket? Found one this morning."
His ticket had been picked up by Cameron Montgomery, a graphic designer who asked at the station who might have dropped it before using Facebook, Linked In and finally Twitter to find the owner.
His "found a ticket" message on the site was retweeted dozens of times before being spotted by Mr Boag ten hours later, the Evening Standard reported.
One email and ten minutes later, owner and ticket were reunited. Mr Montgomery was rewarded with a bottle of champagne.
Mr Boag, a Canadian, was able to enjoy the first night of athletics in the Olympic stadium and said it was "unbelievable". He said the episode had restored his faith in humanity.
"I was massively thankful he thought to put it on Twitter. Someone could have just used the ticket to get in themselves, though they would have had to sit with my friends.
"For him to take the time to try to reunite me with my ticket, it is brilliant that people do that."
Mr Montgomery, originally from Australia but now living in Crouch End, said: "I am always losing things, so the shoe was on the other foot for a change. If it happened to you, you would always want someone to do the same thing.
"I thought it was in the spirit of the Olympics. I thought I would do the right thing."
Mr Boag was able to enjoy the first night of athletics in the Olympic stadium and said it was "unbelievable": "We were quite close to the front so the atmosphere was amazing. There was so much going on.