Thailand is a Buddhist country, but Thais are tolerant of other religions and there are large Christian, Hindu and Sikh communities in Thailand. There is even a tiny Jewish community in Bangkok. Thailand's second largest religious community though is Muslim. The Thai government estimates that the Kingdom is more than 90% Buddhist, a little over 4% Muslim with the other religions making up the remaining amount.
I think the government's numbers are a little off. Thailand is certainly overwhelmingly Buddhist, but there are Muslim communities almost everywhere. Bangkok has several large Muslim communities. The three southern provinces are Muslim majority. Western Thailand, along the border with Myanmar (Burma) is home to a sizable Muslim minority. I wouldn't be surprised if Muslims make up just over 10% of Thailand.
I traveled to southern Thailand for the beginning of the Muslim holy month of Ramadan and then criss-crossed Bangkok photographing Ramadan in Thailand's capital city.