The 34-year-old was found dead at an address in Billingsley, near Alastair Ralph Racing between Bridgnorth and Kidderminster, which he used throughout his victories before becoming an assistant.
West Mercia Police confirmed they were called on Tuesday morning and are not looking for anyone else in connection with the death, which is being treated as unexplained.
Treadwell sprang one of the biggest surprises in the Grand National 11 years ago by riding 100/1 outsider Mon Mome to victory on his first attempt.
He was also third on Monbeg Dude in 2015 and rode at Ludlow on numerous occasions.
A statement from West Mercia Police said: "Earlier this morning police were called to an address in Billingsley, near Bridgnorth, following the death of a man in his 30s.
“The death is currently being treated as unexplained. However, at this stage there is believed to be no third-party involvement.”
Other big successes for Treadwell included the Grand Sefton Handicap Chase at Aintree on Bennys Mist in 2015 and the Byrne Group Plate on Carrickboy at the Cheltenham Festival in 2013.
Treadwell announced his retirement from riding in February 2018 but returned to the saddle in March 2019.
Tributes have already started to pour in for the established rider.
Nick Rust, chief executive of the British Horseracing Authority, said Treadwell had "cemented his place in racing folklore".
Mr Rust said: "We are devastated to hear the tragic news about Liam Treadwell.
"Liam had a fine career in British racing, riding over 300 winners in the 17 years that he held a jockeys' licence, obviously none more memorable than his remarkable 100/1 Grand National success on Mon Mome in 2009 – a ride that cemented his place in racing folklore.
"The racing community will be united in grief today, and the thoughts of everyone at the BHA are with Liam's friends, family and colleagues."
A statement from Aintree Racecourse said: "We’re deeply saddened to hear about the passing of 2009 Grand National winning jockey, Liam Treadwell.
"Our sincere condolences to his family and friends at this difficult time."
Ascot Racecourse added: "Everyone at Ascot is saddened to hear the news that Liam Treadwell has died. Our thoughts are with his family and friends."