The team reached the grain store on Sunday. It is in the Red Sea Mills silos just outside the port town of Hodeidah near a front line area in Yemen's four-year-old civil war.
"We lost access to this mill in September of last year," Stephen Anderson, the World Food Programme (WFP) Yemen country director told Al Jazeera from Djibouti.
"We managed to first gain access, despite repeated attempts in late February and at that time we could see that the grain was in an advanced stage of infestation," Anderson said.
An assessment at that time concluded that about 70 percent of the wheat may be salvageable. The WFP-led team is to begin work to save it.
The war in Yemen has caused the world's largest humanitarian crisis, with 24.1 million people - nearly 80 percent of the population - in need of humanitarian assistance, the UN said.
Tens of thousands have been killed, and the country is on the brink of famine.
It will likely take several weeks to mill what can be salvaged from the 51,000 tonnes of grain into flour and distribute it to the Yemeni communities most in need.
"We must have unimpeded access to this mill," Anderson said. "We are scaling up to helping 12 million people a month so every bit of grain we can get is vitally needed at this time."
Saudi Arabia and the United Arab Emirates are leading the military coalition in war against Yemen.