This study used the Programme for International Assessment of Adult Competencies (PIAAC) dataset to examine informal literacy learning’s effects on adults’ literacy proficiency. Also, the factors associated with informal literacy learning at and outside of work were studied. The study participants were Nordic adults aged 35–65 years. The statistical method was regression analysis, and the results indicate that informal literacy activities at work are associated primarily with occupation, and informal literacy activities outside of work with education, parents’ education and gender. Initial education, occupation, language background and age exerted the strongest estimated associations with reading literacy proficiency. Informal learning, particularly reading outside of work, exerted a statistically significant effect independent of adults’ backgrounds, indicating that it may offer all adults the opportunity to develop literacy proficiency.