Fragile X syndrome (FXS), the most common inherited form of intellectual disability and prevailing known genetic basis of autism, is caused by an expansion in the Fmr1 gene that prevents transcription and translation of fragile X mental retardation protein (FMRP). FMRP binds to and controls translation of mRNAs downstream of metabotropic glutamate receptor (mGluR) activation. Recent work shows that FMRP interacts with the transcript encoding striatal‐enriched protein tyrosine phosphatase (STEP; Ptpn5). STEP opposes synaptic strengthening and promotes synaptic weakening by dephosphorylating its substrates, including ERK1/2, p38, Fyn and Pyk2, and subunits of N‐methyl‐d‐aspartate (NMDA) and AMPA receptors. Here, we show that basal levels of STEP are elevated and mGluR‐dependent STEP synthesis is absent in Fmr1^KO mice. We hypothesized that the weakened synaptic strength and behavioral abnormalities reported in FXS may be linked to excess levels of STEP. To test this hypothesis, we reduced or eliminated STEP genetically in Fmr1^KO mice and assessed mice in a battery of behavioral tests. In addition to attenuating audiogenic seizures and seizure‐induced c‐Fos activation in the periaqueductal gray, genetically reducing STEP in Fmr1^KO mice reversed characteristic social abnormalities, including approach, investigation and anxiety. Loss of STEP also corrected select nonsocial anxiety‐related behaviors in Fmr1^KO mice, such as light‐side exploration in the light/dark box. Our findings indicate that genetically reducing STEP significantly diminishes seizures and restores select social and nonsocial anxiety‐related behaviors in Fmr1^KO mice, suggesting that strategies to inhibit STEP activity may be effective for treating patients with FXS.