The England winger entered in the 66th minute to a huge ovation after his long absence because of a hamstring injury and, within seven minutes, was taking the acclaim of Gunners fans again after nodding home the second goal at Emirates Stadium.
Mikel Merino scored the 37th-minute opener for second-placed Arsenal, who requires an unlikely collapse by Liverpool if they are to capture a first league title since 2004.
Liverpool host Everton in the Merseyside derby on Wednesday, after which there are eight rounds left in the Premier League.
Rodrigo Muniz grabbed an added-time consolation goal for Fulham.
Saka, who has been sidelined since sustaining his injury on December 21 in a league match at Crystal Palace, is also back in time to face Real Madrid in the Champions League quarter-finals starting next week.
Whether Gabriel Magalhaes will be be available against the European champions remains to be seen. The Brazil centre back went off with an apparent hamstring injury in the 16th minute against the Cottagers.
Elsewhere on Tuesday, Anthony Elanga showed Manchester United what they are missing with one of the goals of the season.
The Sweden winger got hold of possession 20 metres inside his own half in the fifth minute and set off on a solo run directly toward United's goal, first shrugging off backtracking opponent Alejandro Garnacho and then entering the penalty area before shooting left-footed into the bottom corner.
Elanga, who left Malmo at age 11 to move to Manchester with his mother and sisters and was soon recommended to United, celebrated in muted fashion against the team that brought him into the pro ranks before selling him to Forest in 2023.
He is now a key player in Forest's unlikely march towards Champions League qualification for the first time. It won't be the team's first time in Europe's top competition, of course — Forest were famously the European Cup winners in 1979 and '80 under Brian Clough.
Forest, who were battling relegation last season, are 10 points clear of sixth-placed Newcastle United. The top five are likely to qualify for next season's Champions League.
At the other end of the table, Wolverhampton Wanderers beat West Ham United 1-0 thanks to Jorgen Strand Larsen's 21st-minute goal to push them 12 points clear of the bottom three.
