<p>Coherent heterojunctions, quantum wells and multiple quantum wells are needed for high-performance devices; these are generally grown via a dedicated vapour phase epitaxy process. Here we demonstrate the growth of coherent perovskite heterojunctions and quantum wells made of mixed-dimensional perovskites using a solution process. By exploiting the solubility difference of methylammonium (MA⁺) and 4-(aminomethyl)piperidinium (4AMP²⁺), we assemble layered perovskites with different layer numbers. The resulting 4AMP-MA_<i>n</i>–₁Pb_<i>n</i>I_3<i>n+</i>1 materials each with different layer numbers or bandgaps form quantum wells. Heterojunctions and quantum wells made of 4AMP-MA₂Pb₃I₁₀ (<i>n</i> = 3) and 4AMP-MAPb₂I₇ (<i>n</i> = 2) with various barrier thickness are tailored by the solution temperature profile during crystal growth. Multiple quantum wells have been formed by cycling temperature profiles. The planar heterojunction and quantum wells have lattice matching without interfacial defects, and exhibit strong thermal stability. Type I band alignment at the <i>n</i> = 2/<i>n</i> = 3 heterojunction is confirmed by both computation and optical studies. This study opens a new direction for the development of sophisticated perovskite heterojunction and quantum well devices.</p>