The plasticizing effect of N,N-diethyl-3-methylbenzamide (DEET) on poly(l-lactic acid) (PLLA) is analyzed and discussed in this manuscript. DEET is the key active ingredient in many commercial mosquito-repellent formulations, and is also an environment-friendly compound, which well matches the biodegradability and compostability of PLLA. PLLA/DEET blends at various compositions were prepared via melt extrusion. In the analyzed composition range, PLLA and DEET are miscible. Addition of few wt.% of DEET results in considerable reduction of the glass transition temperature of PLLA, proving its efficiency as plasticizer. DEET also affects crystallization kinetics of PLLA, with a delayed nucleation upon melt crystallization, but faster crystal growth rate, due to the enhanced mobility of PLLA chains upon addition of the plasticizer. The reported results on miscibility of the blend components also allow to complete the phase diagram of PLLA/DEET blends, which had been previously reported in the literature only for blends where DEET is the major component.