Staples said it would pay $7.25 per share in cash and 0.2188 of its shares for each Office Depot share.
The offer, which is likely to face close antitrust scrutiny, values Office Depot at $11.00 per share, based on Staples closing price on Feb. 2, the last trading day prior to media speculation that a deal was near.
The offer price is a premium of 44 percent to Office Depot's Monday's close.
Last month, activist investor Starboard Value called for the two companies to merge, saying a combined entity would lead to greater cost savings.
Starboard, known for its aggressive shareholder activism, had a 5.1 percent stake in Staples as of December and boosted its holding in Office Depot to nearly 10 percent the same month.
Staples said on Wednesday that it began talks to buy Office Depot in September. The deal is expected to close by the end of 2015.
Office Depot's shares closed up 21.6 percent at $9.28 on the Nasdaq on Tuesday after the Wall Street Journal reported that the two companies were in advanced talks.