Gough heads to Reading after his contract with the Ospreys was terminated by mutual consent earlier this month.
"It's great to have a player of the stature and experience of Ian onboard. He is a proven Test forward and has been exceptional at the set piece and the breakdown area throughout his career," said London Irish director of rugby Brian Smith.
"His presence and influence around the training ground will be massive. He'll add more experience to our young, talented squad."
Gough spent the past six seasons at the Liberty Stadium having previously played for Pontypridd, Newport and the Dragons.
He made 119 appearances for the Ospreys, winning two RaboDirect PRO12 titles and an Anglo-Welsh Cup during his time in Swansea.
A Grand Slam winner in 2008, Gough last played for Wales in November 2010 when he won his 64th cap against Fiji in Cardiff.
"It's my pleasure to announce that I have signed for London Irish. Very excited to play in the Aviva after 17 years service in Wales," said Gough on Twitter.
"Thank you for all your messages, not one bit of abuse yet either!"