Slow growing oomycete isolates with morphological resemblance toPhytophthorawere obtained from forest streams during routine monitoring for the EU quarantine forest pathogenPhytophthora ramorumin Ireland and Northern Ireland. Internal Transcribed Spacer (ITS) sequence analysis indicated that they belonged to two previously unknown species ofNothophytophthora, a recently erected sister genus ofPhytophthora. Morphological and temperature-growth studies were carried out to characterise both new species. In addition, Bayesian and Maximum-Likelihood analyses of nuclear 5-loci and mitochondrial 3-loci datasets were performed to resolve the phylogenetic positions of the two new species. Both species were sterile, formed chlamydospores and partially caducous nonpapillate sporangia, and showed slower growth than any of the six knownNothophytophthoraspecies. In all phylogenetic analyses both species formed distinct, strongly supported clades, closely related toN.chlamydosporaandN.valdivianafrom Chile. Based on their unique combination of morphological and physiological characters and their distinct phylogenetic positions the two new species are described asNothophytophthora irlandicasp. nov. andN.liriisp. nov. Their potential lifestyle and geographic origin are discussed.