Juglans mandshurica and J. regia hybridise naturally in northern China; these trees were first described as J. regia var sinensis C. de Candolle. Dode, in characteristic fashion, treated them as a full species, J. sinensis (C. de Candolle) Dode. Rehder later recognised their hybrid origin, publishing the name J. × sinensis (C. de Candolle) Rehd.
Hybrids between Japanese forms and J. regia also exist. The name J. × notha Rehd. was given to J. ailantifolia × J. regia; the former species included trees treated here as J. mandshurica var. sachalinensis and var. cordiformis.
Both are occasionally cultivated and could easily be mistaken for attractive, large leaved forms of J. regia. At least some have unusual obovate nuts (Grimshaw 2004).
A 37-year-old specimen (labelled J. × notha) at the Sir Harold Hillier Gardens, Hampshire, was measured at 12 m × 39 cm in 2017 (The Tree Register 2019).