Late 16th century; earliest use found in Edmund Spenser (?1552–1599), poet and administrator in Ireland. From reward + -ful. Compare earlier rewarding.
Definition of rewardful in US English:
rewardful
adjectiverɪˈwɔːdf(ʊ)l
That yields or produces reward; rewarding.
Origin
Late 16th century; earliest use found in Edmund Spenser (?1552–1599), poet and administrator in Ireland. From reward + -ful. Compare earlier rewarding.