In the germline, DNA methylation is globally erased in primordial germ cells (PGCs), enabling establishment of sex-specific methylomes in prospermatogonia or oocytes. The catalytic activity of TET1 is required for complete demethylation in PGCs, yet sperm from Tet1 ^-/- mice display methylation defects not explained by incomplete erasure. Instead, these defects arise from abnormal de novo methylation during development, coinciding with erosion of H3K4me3, a chromatin modification that blocks DNMT3A/3L. Using a catalytically inactive Tet1 ^HxD mouse line, we demonstrate a noncatalytic role of TET1 in promoting H3K4me3 deposition and protecting a subset of sperm hypomethylated regions from aberrant methylation in prospermatogonia.