A functional integral algorithm for the partition function and for the temperature Green’s function generating functional of spin 1/2 systems is developed. Starting from the representation of the spin operators in terms of fermion creation and destruction operators we write the above mentioned objects as functional integrals over an infinite dimensional Grassmann algebra. We show how to eliminate the integration over the Grassmann algebra in favor of an integration over function space. The use of the formalism is illustrated with two simple examples.