Illness perceptions (IPs) encompass opinions regarding the nature, severity and curability of a disease. The aim of this cross-sectional study was to investigate the association between alexithymia and IPs among persons with multiple sclerosis (PwMS) and their partners, as well as within the dyads composed of PwMS and partners. PwMS referred to the Multiple Sclerosis Center of the University Hospital “Policlinico-San Marco” from 11th August 2021 to 7th January 2022 and their partners completed a battery of questionnaires, including the Toronto Alexithymia Scale-20 and the Illness Perception Questionnaire Revised. A dyadic data analysis (Actor-Partner Interdependence Model) was performed to test the effect of alexithymic traits both on a person’s own illness perceptions (actor effect) and on the partner’s illness perceptions (partner effect). 100 PwMS (71 women; mean age 47.6 ± 10.4 years) and 100 partners (29 women; mean age 49.1 ± 10.8 years), with a mean partnership duration of 20.1 ± 11.7 years, were enrolled. At the dyadic analysis, statistically significant small-to-moderate actor and partner effects were found considering alexithymia (total score and alexithymic facets) and IPs, whereby higher alexithymic traits related to higher negative perceptions (i.e. consequences, emotional representations) and lower positive ones (i.e. coherence, treatment control). Our findings support the relationship between alexithymia and negative illness appraisals. This data may inform therapeutic interventions aimed at reducing alexithymic traits, which in turn may reduce negative, and potentially dysfunctional, illness perceptions.