In a statement, Yahoo said its board unanimously concluded the proposal was not in the best interests of its shareholders.
The company said the offer "substantially undervalues" its "global brand, large worldwide audience, significant recent investments in advertising platforms and future growth prospects, free cash flow and earnings potential, as well as our substantial unconsolidated investments."
Yahoo added that its board was evaluating all its strategic options.
Microsoft made the half-stock, half-cash offer on February 1. It was originally worth $44.6 billion, or $31 per share -- a 62 percent premium to Yahoo's stock price. Since then, Microsoft shares have fallen, and the deal is now worth $41.5 billion.
Microsoft now must decide whether to sweeten its offer, launch a proxy fight or simply withdraw. Read More>>