Antonyms of adj doubtful

3 senses of doubtful

Sense 1
doubtful, dubious, dubitable, in question (predicate) -- (open to doubt or suspicion; "the candidate's doubtful past"; "he has a dubious record indeed"; "what one found uncertain the other found dubious or downright false"; "it was more than dubitable whether the friend was as influential as she thought"- Karen Horney)

INDIRECT (VIA questionable) -> unquestionable -- (incapable of being questioned; "unquestionable authority")

Sense 2
doubtful, dubious -- (fraught with uncertainty or doubt; "they were doubtful that the cord would hold"; "it was doubtful whether she would be admitted"; "dubious about agreeing to go")

INDIRECT (VIA uncertain, unsure) -> certain, sure -- (having or feeling no doubt or uncertainty; confident and assured; "felt certain of success"; "was sure (or certain) she had seen it"; "was very sure in his beliefs"; "sure of her friends")
INDIRECT (VIA uncertain, unsure) -> certain, sure -- (having or feeling no doubt or uncertainty; confident and assured; "felt certain of success"; "was sure (or certain) she had seen it"; "was very sure in his beliefs"; "sure of her friends")

Sense 3
doubtful, tentative -- (unsettled in mind or opinion; "drew a few tentative conclusions")

INDIRECT (VIA unsettled) -> settled -- (established or decided beyond dispute or doubt; "with details of the wedding settled she could now sleep at night")

Similarity of adj doubtful

3 senses of doubtful

Sense 1
doubtful, dubious, dubitable, in question (predicate) -- (open to doubt or suspicion; "the candidate's doubtful past"; "he has a dubious record indeed"; "what one found uncertain the other found dubious or downright false"; "it was more than dubitable whether the friend was as influential as she thought"- Karen Horney)
       => questionable (vs. unquestionable) -- (subject to question; "questionable motives"; "a questionable reputation"; "a fire of questionable origin")

Sense 2
doubtful, dubious -- (fraught with uncertainty or doubt; "they were doubtful that the cord would hold"; "it was doubtful whether she would be admitted"; "dubious about agreeing to go")
       => uncertain (vs. certain), unsure (vs. sure), incertain -- (lacking or indicating lack of confidence or assurance; "uncertain of his convictions"; "unsure of himself and his future"; "moving with uncertain (or unsure) steps"; "an uncertain smile"; "touched the ornaments with uncertain fingers")

Sense 3
doubtful, tentative -- (unsettled in mind or opinion; "drew a few tentative conclusions")
       => unsettled (vs. settled) -- (still in doubt; "an unsettled issue"; "an unsettled state of mind")