Synonyms for Forthwith:

adv

punctually (adverb)
at once, right away, swiftly, quickly, speedily, precisely, instantly, sharply, punctually, straight away, expeditiously, immediately, promptly.

adv.

immediately (adverb)
now, suddenly, at once, directly, right away, straightaway, quickly, instantly.
punctually (adverb)
at once, right away, swiftly, quickly, speedily, precisely, instantly, sharply, punctually, straight away, expeditiously, immediately, promptly.
soon (adverb)
expeditiously.

n.

suddenly, on-the-spot, fast. forthwith (noun)
in real time, straight off, right away, instantly, straightaway, directly, immediately, now, at once.
without delay (noun)
time, instant, first off.

Other synonyms:

without delay
instant.
Other relevant words:
directly, fast, instant, now, at once, straightaway, in real time, suddenly, straight off, time, first off.

Usage examples for forthwith

  1. The Brahman forthwith set up a shop for selling sandesa, the fame of which soon drew crowds of customers from all parts of the country. – Folk-Tales of Bengal by Lal Behari Day
  2. But one thing must be promptly done; you must quit Paris, forthwith and retire for a time to some quiet retreat, until all this trouble is forgotten. – Captain Fracasse by Theophile Gautier
  3. Forthwith the whole thing must start off ploughing across the lake just like a live mountain sheep. – Down the Columbia by Lewis R. Freeman