The belief in a universal God is a common feature of almost all major religions of the world. This Supreme Divine authority is Omniscient, Omnipotent, Omnipresent, Eternal, Creator, Sustainer, Destroyer, Changeless, Merciful and the Judge. The clear concept of God paves the way to worldly pleasure and salvation in the hereafter. Both Islam and Sikhism believe in the concept of one God where God in Islam, generally known as- “Allah” and in Sikhism- “Waheguru”. Regarding the monotheistic idea sometimes it is considered that Islam has a great and strong influence on Sikhism. This paper is an honest and sincere effort to explain the concept of monotheism and oneness in Islam and Sikhism along with some clarification of the words and terms related with this idea as- Monotheism, Islam, and Sikhism etc. Some people think that Sikhism is an offshoot of both Islam and Hinduism. This article will show that Sikhism is itself a unique religion establishing some unique features. There is an effort to dispel misunderstanding and superstitious beliefs on the concept of God in both Islam and Sikhism and finally to establish peace and harmony among the followers of these two religions.