The single dominant gene Mo3 from accession PIVT1398 of Lactuca virosa was shown previously to confer resistance to all tested isolates of Lettuce mosaic virus (LMV; genus Potyvirus), including mo1‐resistance‐breaking isolates. Because accession PIVT1398 is also resistant to the recently described potyvirus lettuce Italian necrotic virus (LINV), this study checked whether the Mo3 resistance locus could be involved in resistance to LINV. Introgression lines of Mo3 into two different L. sativa genetic backgrounds (Girelle and Mantilia) were resistant to isolates LMV‐9, LMV‐13 and LINV‐Fr (a French isolate of LINV), whereas Girelle and Mantilia were susceptible to these virus isolates. The resistance reaction in inoculated leaves of the introgression lines varied from absence of symptoms and absence of virus detection for LMV‐9, to hypersensitive‐like necrotic lesions and low virus titres for LINV‐Fr and large necrotic lesions and relatively abundant virus detection for LMV‐13. At the systemic level, the resistance phenotype was growth stage‐dependent for LINV‐Fr and LMV‐13. Inoculation of ≥28‐days‐old plants led to absence of symptoms and no virus with the three tested potyvirus isolates in almost all plants. Inoculation of younger plants (≤22‐days‐old) led to chlorotic/necrotic lesions on some apical leaves with low virus titre in a varying proportion of plants with LINV‐Fr and on all plants with efficient virus detection with LMV‐13. These results show that the Mo3 gene, or a tightly linked locus, in L. virosa PIVT1398 controls resistance to LINV in addition to LMV and could be used to breed L. sativa cultivars with broad‐spectrum potyvirus resistance.