DeGale, who started fighting professionally this year, will become a brand ambassador for the heritage boxing marque. Boxers from Mohammed Ali to Ricky Hatton and Joe Calzaghe have all worn Lonsdale gear.
Sunny Singh, sponsorship manager at Lonsdale, said: "James is a massive talent with great charisma. He is already a role-model to many and we look forward to supporting him with our extensive experience in boxing as he punches his way to more success in future."
Lonsdale is among a raft of sportswear brands, which also includes Slazenger, to have been purchased by sporting goods retailer Sports Direct, which today announced pre-tax profits of £57.8m for the half year to 25 October.
The figure was down 41% year on year, while revenues climbed 10% to £756.9m. The retailer upgraded its sales forecasts for the year to April to £155m on the back of robust first half trading.
Sports Direct's majority shareholder and Newcastle United owner Mike Ashley recently renamed St James' Park as Sportsdirect.com @ St James' Park, prompting a backlash from fans.