Gay football fans had raised concerns that they would not be able to visit Qatar for the event as the country punishes gay sex with up to five years in prison.
Mr Cameron was speaking alongside Qatari prime minister, Hamad bin Jassim bin Jabr al Thani when he was asked about the issue.
Press Association reports that he said: “Football is for everybody – no one should be excluded on the basis of their race or religion or sexuality.
“It is absolutely vital that is the case. I am sure that will be the case when the World Cup comes here.”
Mr Cameron continued: “Football can be a great engine for social change and a change of attitudes – we saw in our own country how football drove racism out of the stands.
“Just as that has happened, so too, we need to make sure that there is no place for homophobia in football.”
FIFA president Sepp Blatter had to apologise in December for suggesting that gay fans could visit Qatar if they refrained from sex.