Ferroelectric materials are used in applications ranging from energy harvesting to high-power electronic transducers(1). However, industry-standard ferroelectric materials contain lead, which is toxic and environmentally unfriendly(2). The preferred alternative, BaTiO3, is non-toxic and has excellent ferroelectric properties, but its Curie temperature of similar to 130 degrees C is too low to be practical(3). Strain has been used to enhance the Curie temperature of BaTiO3 (ref. 4) and SrTiO3 (ref. 5) films, but only for thicknesses of tens of nanometres, which is not thick enough for many device applications. Here, we increase the Curie temperature of micrometre-thick films of BaTiO3 to at least 330 degrees C, and the tetragonal-to-cubic structural transition temperature to beyond 800 degrees C, by interspersing stiff, self-assembled vertical columns of Sm2O3 throughout the film thickness. The columns, which are 10 nm in diameter, strain the BaTiO3 matrix by 2.35%, forcing it to maintain its tetragonal structure and resulting in the highest BaTiO3 transition temperatures so far.