The campaign over Initiative 522 drew millions of dollars from out of state and was one of the costliest initiative fights in state history.
The measure was failing 46 per cent to 54 per cent after more ballots were counted Wednesday evening, with the "yes" side trailing by almost 100,000 votes.
Had voters approved I-522, Washington would've been the first state to put in place labeling requirements for genetically modified foods. The opposition raised $22 million to defeat I-522.
Supporters say consumers have the right to know what's in the food they buy, while opponents say the measure would lead to higher food costs.