Two poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N- isopropylacrylamide-co-acrylic acid) (PEP-PEO-P(NIPAm-co-AA)) triblock terpolymers were synthesized by a combination of anionic and RAFT polymerizations, followed by acid hydrolysis. Micellar aggregation and gelation behavior in aqueous solutions were studied by dynamic light scattering (DLS) and rheology, respectively. DLS measurements on dilute solutions revealed that the triblock terpolymers form micelles with PEP cores and PEO-P(NIPAm-co-AA) coronae at room temperature and undergo a micelle to micellar aggregate transition upon heating. Rheological measurements showed that micellar aggregation manifests itself as gelation at higher concentrations (∼4 wt %). The observed thermoresponsive aggregation and gelation is due to the intermicellar association of P(NIPAm-co-AA) blocks in the coronae above the lower critical solution temperature of the P(NIPAm-co-AA) block. The critical micellar aggregation and gelation temperatures are controlled by the mole fraction and degree of acrylic acid (AA) ionization in the P(NIPAm-co-AA) block, and therefore they can be modulated as functions of both pH and AA content in the polymer.