L'anneau de cohomologie de toutes les vari\'et\'es de Seifert
The cohomology ring with coefficients in $\Z_p$, where $p$ is a prime integer, of a Seifert manifold $M$, orientable or not orientable is obtained from a simplicial decomposition of $M$. Many choices must be made before applying Alexander-Whitney formula to get the cup-products. The most difficult choices are those of liftings from the cellular complex to the simplicial complex when we add the condition to be cocycles.