The present invention relates to salt forms of the phosphoinositide 3-kinase (PI3K) inhibitor (S)-7-(1-(9H-purin-6-ylamino)ethyl)-6-(3-fluorophenyl)-3-methyl-5H-thiazolo[3,2-a]pyrimidin-5-one, pharmaceutical compositions comprising the same, and methods of using the salts and compositions for the treatment of PI3K-associated diseases such as cancer.