In two experiments, this article investigates the predictive processing of gender agreement in adult second language (L2) acquisition. We test (1) whether instruction on lexical gender can lead to target predictive agreement processing and (2) how variability in lexical gender representations moderates L2 gender agreement processing. In a pretest-posttest design, Experiment 1 trained 34 intermediate first language (L1) English learners of German on gender assignment. After training, the L2 group showed predictive gender processing; yet, performance correlated with accuracy in gender assignment. Experiment 1 suggests that target knowledge of lexical gender in the L2 lexicon is a prerequisite for predictive use of gender agreement in L2 syntax: Non-target gender assignment would lead to partially erroneous gender prediction such that use of gender agreement is costly for the parser and therefore abandoned. To test this account, Experiment 2 investigated predictive processing in 42 German native speakers who have target-like gender assignment and agreement. In a between-group design, one group received target input and the other received filler items with non-target gender assignment. The latter group subsequently stopped using gender agreement predictively in all experimental trials. Hence, L2 problems with gender agreement can be emulated in native processing. Taken together, the experiments suggest that variability of lexical gender assignment affects processing of gender agreement in natives and non-natives. We interpret the findings in the context of current probabilistic theories of implicit learning and processing adaptation.