synchronicitous

Back-formation from synchronicity: synchronicity +‎ -ous. Attested since the late 20th century.

synchronicitous (not generally comparable, comparative more synchronicitous, superlative most synchronicitous)

1. Of or pertaining to synchronicity; (usually, more particularly) demonstrating synchronicity or apparent synchronicity.

Whereas the noun chronicity corresponds to the adjective chronic, and the noun felicity corresponds to the adjective felicitous, a dedicated adjective to correspond with the noun synchronicity could not well be either synchronic or synchronous, because those words already had other well-established senses. The next natural back-formation candidate after those words was synchronicitous (analogous to felicity/felicitous), and it is therefore unsurprising that that adjective is long since attested.