During his hiatus, it was reported that Mysterio wanted to leave WWE, but the promotion had extended his contract without his approval. Mysterio did not return to WWE programming, and instead appeared in a video message at AAA's Triplemanía XXII and also visited Lucha Underground. On February 26, 2015, WWE officially announced that Mysterio's WWE contract had expired, ending his nearly 13-year tenure with the company.
On March 3, 2015, five days after being released from WWE, it was announced that Mysterio had agreed to work for Lucha Libre AAA Worldwide for the first time since 1995. He made his in-ring return as part of AAA's 2015 pay-per-view Rey de Reyes ("King of Kings") on March 18, 2015. Mysterio teamed up with Myzteziz (formerly Sin Cara in WWE) to defeat the Los Perros del Mal team of El Hijo del Perro Aguayo and Pentagón Jr., with Mysterio scoring the winning pin. This match is notorious for showing wrestler Perro Aguayo Jr.'s death in the ring following a drop kick from Mysterio. Mysterio delivered a dropkick to Perro's back, causing him to fall to the second rope, setting him up for Mysterio's signature "619" wrestling move, which involve Mysterio running towards the rope, grabbing it, and spinning around 180°, hitting the person on the face with his legs. Mysterio, seeing that something was wrong due to Perro becoming limp, purposely missed. The other wrestlers continued performing while each coming up and checking on Perro. They quickly finished the match after realizing that something serious had happened. The ref, wrestlers, and lockerroom crew came out and took Perro out using a piece of plywood. He was pronounced dead by the doctors later that night. His official death was ruled as cardiac arrest.
On May 24, 2015, Mysterio came together with Myzteziz and El Patrón Alberto to form the "Dream Team" for AAA's Lucha Libre World Cup. The trio eventually won the tournament, defeating Johnny Mundo, Matt Hardy and Mr. Anderson in the finals with Mysterio pinning Mundo for the win. For Triplemanía XXIII, AAA's biggest show of the year, a "dream match" between Mysterio and Myzteziz took place. Both wrestlers teamed during their time in WWE and AAA but had never competed against each other. Mysterio was victorious, forcing Myzteziz to submit to a Fujiwara armbar. After the match, Myzteziz turned rudo and sprayed mist at Mysterio's face and challenged him to a Lucha de Apuestas. In early February 2016, AAA announced that Mysterio had left AAA due to financial issues between Mysterio and AAA. Despite not working directly for AAA, Mysterio Jr. participated in the 2016 Lucha Libre World Cup alongside Dr. Wagner Jr. and Dragon Azteca Jr., known as "Team Mexico International" the trio finished in third place.
Mysterio signed with Lucha Underground on December 12, 2015 and appeared on the second season. On January 10, 2016, Mysterio teamed with Dragon Azteca Jr. and Prince Puma to defeat Ivelisse, Johnny Mundo and Son of Havoc and Fénix, Jack Evans, and PJ Black and Cortez Castro, Joey Ryan and Mr. Cisco in a 4-way Trio Tag Team Elimination Match for the Lucha Underground Trios Championship. Mysterio was the second competitor to enter the second ever Aztec Warfare match and was the final elimination by the eventual winner Matanza. On January 31, 2016, at Ultima Lucha Dos, Rey Mysterio defeated Prince Puma in a singles match. Mysterio defeated Chavo Guerrero Jr. in a Loser Leaves Match. On April 9, 2016 Mysterio was in an Aztec Warfare match where he eliminated Matanza Cueto after Mysterio was eliminated by Johnny Mundo.
On March 20, 2015, Mysterio partnered with Extreme Tiger in a match against the tag team of Perro Aguayo Jr. and Manik. Mysterio scored the winning pin on Manik while all involved were unaware that Aguayo was dead (rather than just knocked out) following a dropkick to Aguayo by Mysterio to the middle rope. Mysterio then attempted the 619 on Aguayo and Manik, but did not connect. Aguayo was rushed to a hospital and doctors attempted to revive him, officially pronouncing him dead at 1:00 AM on March 21.