We report the highly efficient synthesis of sub-2-nm diameter single-walled carbon nanotube (SWCNT) forests with homogenous and controllable diameter (1.5–2.8 nm), millimeter-scale height, and extremely high purity. This control was achieved by combining a previously reported sandwich catalyst (Al/Fe/Al) with an atmospheric synthesis process and the water-assisted thermal chemical vapor deposition method. In this way, a dense array of small and stable catalyst nanoparticles, suitable to support vertical growth, while avoiding gas diffusion limitations, could be prepared from which SWCNT forests could be synthesized efficiently to achieve both high purity and mm-scale height. The mm-tall sub-2-nm diameter SWCNT forests showed good quality (Raman G/D ratio of ∼40) and exceptionally high as-grown purity (outer specific surface area of 1215 m2/g; ideal: 1315 m2/g), which can be directly used without additional post-growth purification process to avoid any potential damage to the CNTs. This result represents a significant advance in synthesizing highly pure, mm-scale length SWCNT forests with sub-2-nm diameters.