Sequence homology between SARS-CoV-2 and common-cold human coronaviruses (HCoVs) raises the possibility that memory responses to prior HCoV infection can impact the T cell response in COVID-19. We studied T cells recognizing SARS-CoV-2 and HCoVs in convalescent COVID-19 donors, and identified a highly conserved SARS-CoV-2 sequence S_811-831, with two overlapping epitopes presented by common MHC-II proteins HLA-DQ5 and HLA-DP4. These epitopes were recognized by CD4⁺ T cells from convalescent COVID-19 donors, mRNA vaccine recipients, and by low-abundance CD4⁺ T cells in uninfected donors. TCR sequencing revealed a diverse repertoire with public TCRs. CD4⁺ T cell cross-reactivity was driven by the remarkably strong conservation of T cell contact residues in both HLA-DQ5 and HLA-DP4 binding frames, with distinct patterns of HCoV cross-reactivity explained by MHC-II binding preferences and substitutions at secondary TCR contact sites. These data highlight S_811-831 as a highly-conserved CD4⁺ T cell epitope broadly recognized across human populations.