Magicseaweed, founded in 2002 in Kingsbridge, provides forecasting and live reporting of more than 4,000 beaches around the world.
Surfstitch purchased the firm as part of a combined deal that also includes surf magazine Stab.
Magicseaweed said it was "thrilled" as the deal would enable it to expand.
The firm employs 22 people in Kingsbridge, taking data from offshore weather buoys to forecast surfing conditions, along with allowing surfers to check conditions via web cameras on beaches.
"It's a unique opportunity to find the right balance of surf forecasting, inspirational content and product offerings to our global surf community and millions of users," said co-founder Ryan Anderson.
Justin Cameron, chief executive of Surfstitch, said: "We are excited to welcome Magicseaweed and Stab to the Surfstitch Group.
"These businesses share our enthusiasm and passion in the action sports and youth culture space, and are ideal partners to support Surfstitch's mission to become the global destination for action sports and youth lifestyle content and online retail."

The painting, depicting the 1654 ceremony, had hung in Rokeby Park, Barnard Castle, since 1880, according to its owner Sir Andrew Morritt.
But it became damaged. Now after being restored, Sir Andrew is trying to find out how it ended up in Durham.
Experts have studied it, but do not know who painted it.
It has also not yet been valued.
Sir Andrew said: "What is a mystery is when it came and where it came from, or who it was painted by.
"All I know for a fact is it has been here since at least 1880, and probably longer."
Veronique Gerard-Powell, chief curator and senior lecturer at the Sorbonne, in Paris, said: "This is the only painted representation of the coronation. There is no other like it - it really is quite unique."
Rokeby Park, which was built between 1725 and 1735, has hundreds of valuable paintings, according to Sir Andrew.

The man was injured by accident on "soft ground" near Okehampton in Devon, police said.
The man was with his 10-year-old daughter the time, officers said. There were no reports she was injured.
It is not known how extensively the man was injured, but Sgt Harry Tangye, of Devon and Cornwall Police, tweeted that the early signs were "good".
An air ambulance was called to the scene after the incident on Tuesday morning.

Georgina Callander, 18, was among 22 people who died in the suicide bombing at Manchester Arena on 22 May.
Hundreds attended the ceremony at Holy Trinity Church in Tarleton, Lancashire.
Attendees wore "a touch of yellow" at the request of her family, who thanked mourners for their "sympathy and support".
Parents Lesley and Simon, with brothers Daniel and Harry, laid flowers in the village before the funeral.
Shops were closed as a mark of respect as the funeral cortege made its way through.
Bishop John Goddard said the service was a chance to "put aside hatred and look to embrace love and hope".
A private burial service was held after the church ceremony.
Ms Callander was described as "beautiful" by friends who said on social media that she was one of the first people taken to hospital after the attack.
She was said to be a huge fan of 23-year-old Grande and was pictured with her in 2015.
The final song played at the service was the US singer's hit One Last Time, which has become an anthem for fans since the attack.
Grande performed an emotional rendition of the song at the One Manchester benefit concert on 4 June.
Devon-based surfing goods and forecasting firm Magicseaweed has been bought by an Australian company as part of a £7m deal.
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A country house owner is trying to find out how the only known painting of the coronation of the French king Louis XIV end up in his stables.
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Streets of a village were decorated with pink and yellow ribbons for the funeral an Ariana Grande "superfan" killed in the Manchester terror attack.