Abstract. Let F denote a field, and fix a nonzero q ∈ F such that q 4 = 1. We define an associative F-algebra ∆ = ∆ q by generators and relations in the following way. The generators are A, B, C. The relations assert that each ofis central in ∆. We call ∆ the universal Askey-Wilson algebra. We discuss how ∆ is related to the original Askey-Wilson algebra AW(3) introduced by A. Zhedanov. Multiply each of the above central elements by q + q −1 to obtain α, β, γ. We give an alternate presentation for ∆ by generators and relations; the generators are A, B, γ. We give a faithful action of the modular group PSL 2 (Z) on ∆ as a group of automorphisms; one generator sends (A, B, C) → (B, C, A) and another generator sends (A, B, γ) → (B, A, γ). We show that {A i B j C k α r β s γ t |i, j, k, r, s, t ≥ 0} is a basis for the F-vector space ∆. We show that the center Z(∆) contains the elementUnder the assumption that q is not a root of unity, we show that Z(∆) is generated by Ω, α, β, γ and that Z(∆) is isomorphic to a polynomial algebra in 4 variables. Using the alternate presentation we relate ∆ to the q-Onsager algebra. We describe the 2-sided ideal ∆[∆, ∆]∆ from several points of view. Our main result here is that ∆[∆, ∆]∆ + F1 is equal to the intersection of (i) the subalgebra of ∆ generated by A, B; (ii) the subalgebra of ∆ generated by B, C; (iii) the subalgebra of ∆ generated by C, A.