DegP is a highly conserved protease that performs regulated proteolysis to selectively remove misfolded proteins in the periplasm of Escherichia coli Binding of misfolded proteins is known to be the main mechanism of DegP activation, but it is unknown whether any native proteins can alter DegP activity. Here, we show that a small periplasmic protein, YjfN, which is highly upregulated by the Cpx envelope stress response, functions as a "suicide activator" for DegP and promotes efficient degradation of misfolded proteins. YjfN readily binds to and is degraded by DegP, for which a hydrophobic C-terminal residue and transient unfolding of YjfN are critical. YjfN also activates DegP in trans while it is being degraded and accelerates degradation of a denatured outer membrane protein, OmpA, that is not easily recognized by DegP. Although YjfN also prevents OmpA aggregation, the trans-activation effect is mainly responsible for efficient OmpA degradation. Overexpression of YjfN enhances the viability of cells in misfolded protein stress that is induced by the presence of a less-active variant of DegP at high temperature. Collectively, we suggest that YjfN can enhance DegP proteolysis for relieving envelope stresses that may generate toxic misfolded proteins.IMPORTANCE Proper degradation of toxic misfolded proteins is essential for bacterial survival. This function is mainly performed by a highly conserved protease, DegP, in the periplasm of Escherichia coli It is known that binding of misfolded proteins is the main mechanism for activating the DegP protease. Here, we find that a small periplasmic protein, YjfN, can be a substrate and an activator of DegP. It is the first example of a native protein showing an ability to directly alter DegP activity. The YjfN-mediated trans activation of DegP promotes efficient degradation of misfolded proteins. Our results suggest that YjfN is a novel "suicide activator" for DegP that enhances DegP proteolysis under misfolded protein stress.