Marinoff held off Riddell in a thrilling count on Monday night to poll 23 votes out of a possible 33 for the league's highest individual honour.
Essendon midfielder Madison Prespakis, who won the honour in 2020 when she played for Carlton, finished third with 18 votes.
Marinoff is the second Adelaide player to win the award after all time great Erin Phillips, and adds the best and fairest to her seventh All-Australian selection, also announced on Monday.
The award caps a near-perfect season for the three-time premiership midfielder 27-year-old, who was also judged player of the year by AFLW coaches.
Marinoff's triumph was never certain, with Riddell keeping up until the final round.
With just two points the difference, Marinoff clinched the award when Riddell received only one vote for her round 10 performance.
Marinoff iced her evening with an additional two votes.
Despite being named All-Australian captain earlier on Monday night, North Melbourne star Jasmine Garner did not poll any votes until round five.
Marinoff polled 23 votes out of a possible 33 to take the AFLW's highest individual honour. (Con Chronis/AAP PHOTOS)
Garner, who has a surprising a history of not polling well in the women's equivalent of the Brownlow, finished with 12 votes.
She and Marnioff joined North captain Emma Kearney as the only AFLW players to be named in seven or more All-Australians teams.
Marinoff was named vice-captain, while Garner was joined by North teammates Alice O'Loughlin and now four-time All-Australian Riddell.
Adelaide and Brisbane were well represented with four players each, with Crows veteran Chelsea Randall named on the interchange for her fifth nod while Lions captain Bre Koenen missed out after earning a spot in the extended squad.
West Coast's 19-year-old gun Ella Roberts was the youngest player in the squad and one of 12 players to receive their maiden All-Australian blazer.
Eagle Ella Roberts has soared into the side as its youngest member. (Richard Wainwright/AAP PHOTOS)
Fremantle ruck Mim Strom edged out Port teenage sensation Matilda Scholz for the ruck position for her first selection.
Scholz however, claimed the Rising Star award after earning 57 votes out of a possible 60.
The Port ruck also won mark of the year and Adelaide's Hannah Munyard claimed goal of the year.
AFLW BEST AND FAIREST
1. Ebony Marinoff (Adelaide, 23 votes)
2. Ash Riddell (North Melbourne, 20 votes)
3. Madison Prespakis (Essendon, 18 votes)
AFLW RISING STAR VOTING:
1. Matilda Scholz (Port Adelaide, 57 votes)
2. Shineah Goody (Port Adelaide, 48 votes)
3. Amy Gaylor (Essendon, 26 votes)