Objectives: In the present study, we aimed (i) to describe the personality traits of a cohort of post-COVID-19 condition (PCC) patients compared with a healthy control (HC) group, (ii) to evaluate the relationship between sleep quality and personality traits, and (iii) to investigate whether this relationship differs according to disease severity. Methods: We included 599 participants from the Nautilus Project (ClincalTrials.gov IDs: NCT05307549 and NCT05307575) with an age range from 20 to 65 years old. Of 599 participants, 280 were nonhospitalized (mild PCC), 87 were hospitalized (hospitalized PCC), 98 were in the PCC-ICU, and 134 were in the HC group. We assessed sleep quality with the Pittsburgh Sleep Quality Index (PSQI) and personality traits with the NEO Five-Factor Inventory (NEO FFI). Results: We found that mild-PCC patients had higher scores of neuroticism than HCs (p < 0.001) and ICU-PCC patients did (p = 0.020). The higher the neuroticism score was, the higher the total PSQI score (B 0.162; p < 0.001), the worse the sleep latency (B 0.049; p < 0.001), the greater the degree of sleep disturbance (B 0.060; p < 0.001), the greater the use of sleeping medication (B 0.035; p = 0.033), and the greater the incidence of daytime disturbances (B 0.065; p < 0.001) among the PCC patients. High neuroticism is also an indicator of worse sleep quality in mild-PCC (t = 3.269; p 0.001) and hospitalized-PCC (t = 6.401; p < 0.001) patients and HCs (t = 4.876; p < 0.001) but not in ICU-PCC patients. Conclusions: Although neuroticism affected sleep quality in both the PCC patients and HCs, the clinical implications and magnitude of the relationship were more significant in the PCC group. Specific and multidimensional interventions are needed to treat sleep problems in this population, and the influence of their personality traits should be considered.