Sports Direct, the retailer owned by billionaire Newcastle United owner Mike Ashley, has purchased 4.6% of Debenhams shares.
Sports Direct acquired 56.8 million shares in the department store, it told the London Stock Exchange on Monday.
Debenhams shares fell sharply at the end of 2013, and it has warned Christmas trading was not matching its expectations and profits would be hit.
Sports Direct joined the FTSE 100 in September 2013.
Debenhams share price, which had recovered some of its lost ground by the end of last week, rose more than 5% in early trading after the announcement from Sports Direct.
The department store chain, which has 240 stores in 28 countries, issued a profit warning on the last day of 2013, saying that it now expects pre-tax profits for the six months to April 2014 to be £85m, down from £115m in the same period the previous year.