This report shows that the size, shape, and composition of presynthesized copper nanoparticles can be nanoengineered through exploiting concurrent interparticle aggregative growth and interfacial carbon−sulfur cleavage in a thermally activated evolution route. This is demonstrated by thermally activated processing of ultrafine copper nanoclusters encapsulated with thiolate monolayer (Cun(SR)m) toward semiconducting copper sulfide (Cu2S) nanodiscs with controllable sizes and shapes. Under controlled temperatures (120−150 °C), the ultrafine Cun(SR)m nanoclusters, with a size of 0.5 nm evidenced by TEM, SAXS-WAXS, DCP-AES, and MALDI-TOF measurements, were shown to evolve into thiolate-capped Cu2S nanodiscs via thermally activated coalescence and copper-catalyzed interfacial C−S cleavage reactivities. The Cu2S nanodiscs, as confirmed by XPS and HRTEM analyses, exhibited controllable and monodispersed sizes depending on the thermal processing parameters, ranging from 5 to 35 nm in the disk dimension and 3−6 nm in the thickness dimension. These nanodiscs are stable and display remarkable 1D/2D ordering upon self-assembly. This process is not a simple digestive ripening of smaller particles because it involves an aggregative nucleation and growth process distinctively different from traditional ripening and a reactive carbon−sulfur bond cleavage controlled by the catalytic effect of copper under the specified temperatures. The coupling of the thermally activated coalescence and C−S bond cleavage to convert the ultrafine Cu nanoclusters toward the formation of Cu2S nanodiscs is highly effective for tuning nanoscale size, shape, and composition, and could find applications in nanoengineering a variety of semiconducting nanocrystals for applications in nanostructured electronic, sensing, and photochemical devices.