This study examines polymorphism of second person singular forms of address (2PS) in the Spanish of Medellin, Colombia. Polymorphism of 2PS is the usage of more than one 2PS directed toward the same person during the same interaction. Speakers from Medellin (N = 72) carried out an oral discourse completion task that consisted of 24 situations. Results showed that polymorphism is common in Medellin Spanish as it occurred 22.7% of the time. It was also found that polymorphism is constrained by three variables: change in speech act, speaker sex, and interlocutor relationship. In particular, a change in speech act, male speakers, and intimate interlocutors favored polymorphism. In addition, characteristics of polymorphism are identified. Findings demonstrate that polymorphism is not random but rather systematic and purposeful—it allows speakers to express different speech acts and emotions via the shift in 2PS. Results are discussed in light of broader implications and the future of polymorphism in the Spanish of Medellin. •Polymorphism of second person singular forms of address is typical of paisa Spanish.•Polymorphism of second person singular forms of address is systematic.•Polymorphism can be explained by changes in speech act.•Polymorphism is a purposeful tool that speakers utilize.•Polymorphism is expected to be maintained in the Spanish of Medellin.