But a further $42.5 million is still needed by the UN's World Food Programme to provide food vouchers to almost 1.7 million Syrians who have fled to Jordan, Lebanon, Turkey and Egypt up until the end of the month.
The agency warned on Monday that it was suspending the distribution of vouchers after it ran out of money, blaming donors for failing to honour their promises.
It launched a campaign on social media on Wednesday to raise $64 million (51 million euros) to keep the programme running, supported by Aloe Blacc, who released the hit song "I Need A Dollar" in 2010.
WFP spokeswoman Elisabeth Byrs told AFP that more than 10,000 individuals, corporations and governments around the world contributed $21.5 million in the first 24 hours.
And she confirmed this meant the food programme was continuing for refugees outside Syria.
The campaign, #ADollarALifeline, is based on the idea that donating even $1 can help if enough people take part. It will continue until 0800 GMT on Saturday.
Signup to our newsletter and follow us on Facebook and Twitter!
Food for four million Syrians inside the country had already been purchased and were therefore unaffected, but Byrs warned that without new sources of funding they could be stopped in February.
The WFP needs a total of $353 million to feed Syrian refugees inside and outside the country during December, January and February, she said.
More than half of Syria's population have been forced to flee their homes since war broke out in March 2011.
Some 3.2 million have fled beyond the country's borders, and more than 7.2 million have become internally displaced, according to the United Nations.
Through its system of food vouchers, the WFP has since 2011 injected $846 million into the economies of Syria's neighbours who have taken in refugees.
This week was the first time it has had to suspend its operations because of a shortage of funds.
There is no end in sight to the fighting in the brutal conflict that has killed nearly 200,000 people.