In mathematics, particularly q-analog theory, the Ramanujan theta function generalizes the form of the Jacobi theta functions, while capturing their general properties. In particular, the Jacobi triple product takes on a particularly elegant form when written in terms of the Ramanujan theta. The function is named after mathematician Srinivasa Ramanujan.

The Ramanujan theta function is defined as

for |ab| < 1. The Jacobi triple product identity then takes the form

Here, the expression ${\displaystyle (a;q)_{n}}$ denotes the q-Pochhammer symbol. Identities that follow from this include

and

and

This last being the Euler function, which is closely related to the Dedekind eta function. The Jacobi theta function may be written in terms of the Ramanujan theta function as:

We have the following integral representation for the full two-parameter form of Ramanujan's theta function: