During the germinal centre (GC) reaction, mature B cells undergo rapid and reversible phenotypic shifts that are essential for adaptive immunity. Here we report that GC B cells, unlike other mature B cells, transiently acquire a unique epigenetic plasticity, demonstrated by their enhanced capacity to reprogram to induced pluripotent stem cells. This plasticity depends on T follicular helper (T_FH) cells and is not due to increased proliferation or MYC activation. Instead, it involves weakening of B-cell identity and derepression of stem and progenitor programs driven by NF-κB and other T_FH-derived signals. Thus, physiological GC plasticity is tightly constrained by the affinity maturation process of positive selection. Loss of histone 1, a chromatin compaction regulator restricting the accessibility of embryonic stem cell programs, further enhances GC plasticity by bypassing this gatekeeping mechanism. Importantly, patients with B-cell lymphoma enriched for GC plasticity signatures had worse outcomes, suggesting that this mechanism may also contribute to lymphomagenesis.