Yes, Muslims believe in Allah(God).
Muslims believe that Allah is the all-powerful Creator of a perfect, ordered universe. He is transcendent and not a part of his creation. He is all-powerful, absolutely Just and He is First, Last, and Everlasting; He was when nothing was, and will be when nothing else remains. Also, There is no other entity in the entire universe worthy of worship besides Him.
This belief in God is how one becomes a Muslim and is similar to the first commandment of Christianity and Judaism: You shall have no other gods before Me.
The Quran presents this belief in Allah as:
Say He is Allah,the One,
Allah the Eternal,Absolute;
He does not beget,not nor is He begotten,
And there is none like Him. -- Al-Ikhlas(1:1-4)
To know more about the fundamental Muslim beliefs and How to convert to Islam, Click Here.