He fared well at his pro day and attracted representatives and scouts from 18 NFL teams as the primary prospect working out at the event.

Head coach Jim Tomsula named Tartt the backup strong safety to begin the regular season, behind veteran Antoine Bethea.

Defensive coordinator Eric Mangini named Tartt the starter after Bethea tore his pectoral muscle the previous week and missed the remainder of the season.

[27] On December 13, 2015, Tartt collected eight combined tackles, deflected a pass, and made his first career interception during a 24–10 loss at the Cleveland Browns in Week 14.

[32] Head coach Chip Kelly named Tartt the backup strong safety behind Antoine Bethea to start the regular season in 2016.

[34] On November 27, 2016, Tartt made his first start at free safety and recorded three combined tackles in their 31–24 loss at the Miami Dolphins in Week 12.

He earned the starting role for the rest of the season after the 49ers placed Eric Reid on injured reserve after he tore his bicep in his right arm.

[38] Defensive coordinator Jim O'Neil held a competition between Tartt and Eric Reid to name a new starting strong safety after Antoine Bethea was granted his release.

[39] Head coach Kyle Shanahan named Tartt the backup strong safety to begin the regular season, behind Eric Reid.

[41] Tartt started the first two regular season games at free safety in place of Jimmie Ward who was inactive due to a hamstring injury.

[44] He finished the 2017 season with 55 combined tackles (38 solo), three pass deflections, an interception, and was credited with half a sack in nine games and nine starts.

[48] In Week 10 against the Seahawks on Monday Night Football, Tartt forced a fumble on rookie wide receiver DK Metcalf which he recovered in the 27–24 overtime loss.

[50] In Super Bowl LIV against the Kansas City Chiefs, Tartt recorded a sack on Patrick Mahomes during the 31–20 loss.

[57] Against the Los Angeles Rams in the NFC Championship, Tartt had six tackles, but dropped a critical would-be interception in the fourth quarter in the 20-17 loss.