This study examined whether the linguistic distance between the first (L1) and second (L2) language is a significant determinant of L2 writing skills of 292 adult immigrants from 39 different source countries, who were beginner learners of Spanish L2. Gender, age, length of residence in Spain, education level as a proxy for literacy skills in L1, enrolment in Spanish language courses, and overall communicative competence in Spanish were also considered. Using both standard procedures for assessing L2 writing and performance-based psycholinguistic measures of accuracy and text-production fluency, the findings highlight the important role of linguistic proximity in achieving greater accuracy, text-production fluency, and overall L2 writing scores. Other significant predictors were age, enrolment in Spanish courses, and education level for accuracy; and length of residence in Spain and education level for text-production fluency. Although length of residence in Spain was negatively associated with text-production fluency in L2 writing, mediation analyses revealed that the effect of age on text-production fluency was mediated by length of residence in Spain and that L2 proficiency level mediated the link between linguistic distance and text-production fluency. Furthermore, most of the errors that these immigrants made were morphosyntactic and spelling errors, while vocabulary errors were rare.