Belgian investment firm GBL was placing a tenth of its 7.5% holding, or 1.37 million shares, at 257.75 euros on the market for sale, a 1.7% discount to Tuesday's closing price, the traders added.
The sale is worth 353.1 million euros.
GBL and Adidas, whose shares hit record highs on Tuesday, declined to comment.
Adidas is in the midst of a share buyback, planning to repurchase 3 billion euros in stock between March 2018 and May 2021, which has gradually pushed up GBL's relative stake.
Adidas share price has more than doubled since GBL invested in the company as CEO Kasper Rorsted focused on improving profitability, expanding in North America and China and boosting online sales.
In GBL's recent annual report, it said it intends not to have a single asset exceeding 15 to 20% of its portfolio. Adidas made up 17.5% of GBL's portfolio at the end of March, with the stock's value surging to record highs this year.
At 0919 GMT, Adidas shares were down 2.6% at 255.4 euros and lagging the broader DAX index, which was down 0.5%.
GBL, founded by Belgium's richest man Albert Frere, who died in December at the age of 92, first invested in Adidas in 2015 with the purchase of a 3% stake.
GBL also holds stakes in some of Europe's biggest companies, including Pernod Ricard, LafargeHolcim and Total.
(Reporting by Thyagaraju Adinarayan; additional reporting by Helen Reid in LONDON, Philip Blenkinsop in BRUSSELS and Emma Thomasson in BERLIN; writing by Josephine Mason; editing by Alexander Smith and Louise Heavens)
By Thyagaraju Adinarayan, Emma Thomasson and Philip Blenkinsop