Onion-like carbons (OLCs) are a class of fullerene-like circular nanoallotropes of carbon, typically synthesized from nanodiamond (ND) via thermal annealing, plasma spraying, and laser ablation. These methods require high temperature, high vacuum, or inert gas. Here, we report an ambient electrospray deposition (AESD) process to transform NDs (11 ± 1 nm in size) into OLCs (50 ± 13 nm in size) in water. Transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used for the characterization of NDs and OLCs. High-resolution TEM images showed an increased interplanar spacing from ND (0.23 nm) to OLC (0.39 nm). Raman spectra showed a shift in the ND peak from 1336 cm-1 to D-band at 1349 cm-1, and XPS quantitatively estimated an increase in the graphitization ratio (sp2/sp3) from 0.95 to 3.16 after AESD. Comparison of electrospray with sonic spray confirmed that such a transformation required an external voltage as well. AESD was also performed for NDs dispersed in ethanol and acetonitrile, which showed a solvent-dependent transformation. © 2021 American Chemical Society.