AbstractFew semiconducting polymers are known that possess more than one semi‐crystalline structure. Guidelines for rationalizing or creating polymorphism in these materials do not exist. Two different semi‐crystalline polymorphs, β1 and β2, and an amorphous α phase have recently been identified for alternating diketopyrrolopyrrole‐quaterthiophene copolymers (PDPP4T). The polymorphs differ structurally by the π–π stacking distance, and electronically by the optical bandgap and charge carrier mobility. Here we investigate the corresponding terthiophene (PDPP3T) derivatives, to study the effect of the relative orientation of adjacent DPP units on the polymorphism. In PDPP3T, the relative orientation of DPP units alternates along the chain, while in PDPP4T it is constant. We show that the two polymorphs, β1 and β2, can also be generated for a PDPP3T polymer in solution and thin film. Interestingly, compared to PDPP4T, more solvents can induce the two distinct semi‐crystalline polymorphs for PDPP3T via a β1 → α → β2 polymorphic transition.