In this study peroxynitrite (ONOO−) is proposed as an important player in defence responses during the interaction of potato (Solanum tuberosum) and the oomycete pathogen Phytophthora infestans. The potato–avr P. infestans model system exhibited a transient programme of boosted ONOO− formation correlated in time with the burst of nitric oxide (NO) and superoxide during the first 6 h post-inoculation (hpi). The early ONOO− over-accumulation was not accompanied by TPx gene expression. In contrast, the compatible interaction revealed a 24 h delay of ONOO− formation; however, an enhanced level of NO and superoxide correlated with TPx up-regulation was recorded within the earlier stages of pathogen infection. Peroxynitrite over-accumulation in the susceptible potato coincided with an enhanced level of protein tyrosine nitration starting from 24 hpi. Surprisingly, the nitroproteome profile of the resistant potato did not show any visible difference after inoculation, apart from one band containing subtilisin-like protease-like proteins, which appeared 48 h after pathogen attack. An additional pharmacological approach showed that treatment of the susceptible genotype with ONOO− followed by inoculation with P. infestans contributed to slowing down of the colonization of host tissues by the pathogen via a faster and stronger up-regulation of the key defence markers, including the PR-1 gene. Taken together, the results obtained indicate that a precise control of emitted NO and superoxide in cooperation with thioredoxin-dependent redox sensors in sites of pathogen ingress could generate a sufficient threshold of ONOO−, triggering defence responses.