Foraminiferal calcite Mg/Ca (Mg/CaCC) is used in paleoceanographic studies to reconstruct temperature. Furthermore, the Mg/CaCC is influenced by different seawater Mg/Ca (Mg/CaSW). Foraminiferal calcite Sr/Ca (Sr/CaCC) can potentially be used to reconstruct Sr/Ca ratios of seawater (Sr/CaSW). As these elements are the most abundant of all elements incorporated into the calcium carbonate of the foraminiferal tests, they potentially might affect each other's incorporation. To investigate the effects of the Mg concentration in the test on Sr incorporation, we conducted a culture study with two species of benthic foraminifera producing carbonate tests with different Mg content. Foraminifers grew under controlled conditions in different Mg/CaSW, whereas Sr/CaSW was kept constant. By analyzing Sr/CaCC of cultured specimens with laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS), we show that Sr/CaCC increases with increasing Mg/CaCC. We explain this observation by a stress in the crystal lattice, caused by elevated Mg/CaCC, which may be compensated for by the additional incorporation of Sr. We discuss this finding in the context of biomineralization and evaluate the reliability of Sr/CaCC as a possible Sr/CaSW proxy.