Although previous research has observed a facilitative influence of the first language (L1) on the acquisition and processing of gender agreement in a second language (L2), particularly in language pairs with similar gender agreement marking, the question of whether knowledge of two languages with gender can confer an additional advantage for L3/Ln (third or additional language) learners has not yet been addressed. The present study aimed to fill this research gap by examining the processing of gender agreement in intermediate and advanced L3/Ln Swedish among two groups of Polish native speakers: 30 L2 English/L3 Swedish learners, and 30 L2 English/L3 German/L4 Swedish learners. Participants were tested by means of a speeded grammaticality judgment task, in which they judged the correctness of indefinite noun phrases that either agreed or did not agree in gender. They also completed an untimed gender assignment task to control for their lexical knowledge of gender. Accuracy and response time data were submitted to Generalized Linear Models. The analysis shows that L4 Swedish learners process noun phrases faster than L3 Swedish learners, but only at the intermediate proficiency level; however, the groups do not differ in their judgment accuracy. This advantage is interpreted in terms of a surface transfer of similar gender agreement marking, which helps the learners automatize gender agreement processes earlier, but does not increase their sensitivity to gender-agreement violations. Moreover, the results accord with previous L2 studies in showing that learners of Swedish as L3/Ln develop sensitivity to ungrammaticality with advancing proficiency and benefit substantially from their gender assignment knowledge in processing gender agreement. Crucially, the present study provides preliminary evidence of a multilingual advantage in processing morphosyntactic features in L3/Ln.