After a prolonged period of successive crises and recession, the Greek labour market is still facing a number of structural challenges, among which skill mismatch is the more pronounced. Greece has the largest share of overqualified graduates in Europe and ranks among the bottom 20% of countries regarding the alignment of skills with the labour market, the intensity of skill use in workplaces and the use of high-performance workplace practices. Skill mismatch can have detrimental effect in an economy reducing its productivity and competitiveness through unemployment and inefficient allocation of resources. Drawing on primary research data retrieved via original questionnaire answered by 642 participants, this study aims to identify determinants of various forms of job mismatch, both vertical and horizontal, in the employment of tertiary graduates in Greece. The results of multinomial logistic regression analysis indicate that employment-related characteristics such as the specialization level of sector and method used to obtain the job position play a significant role in mismatch as well as a number of demographic attributes like age, gender and marital status. In addition, the role of spatial-related labour market characteristics is verified, revealing that people who reside in urban regions over-burdened by the unemployment are more likely to experience job mismatch.