Castaño vs. Vitu

After Demetrius Andrade vacated his WBA (Regular) title, the WBA elevated Castaño to regular champion. Shortly thereafter, the WBA ordered a fight between Castaño and WBA (Super) champion, Erislandy Lara.^[9] Castaño would first face Cedric Vitu on 10 March 2018. Castaño showcased his talent, and defeated Vitu via a 12th-round TKO, dominating him from start to finish to retain his WBA (Regular) title.^[10]

Castaño vs. Lara

On 2 March 2019, Castaño defended his title against former world champion Erislandy Lara. The fight ended in a split draw, one judge scoring it 115–113 for Castaño, one judge scoring it 115–113 for Lara while the third had it 114–114.^[11][12]

On June 25, 2019, the WBA announced that they are stripping Castaño of his belt, because he did not sign the contract for a rematch against Cedric Vitu, his immediate mandatory challenger.^[13]

In his next fight, Castaño faced veteran Wale Omotoso. The fight was a one-sided affair in favor of Castaño, up until the end of the fifth round, in which Omotoso declared a shoulder injury was too painful to proceed, awarding Castaño with the victory.^[14]

Castaño vs. Teixeira

On 13 February 2021, Castaño defeated WBO light middleweight champion Patrick Teixeira via wide unanimous decision with judges' scores of 119–109, 120–108 and 117–111 in his favor.^[15]

Castaño vs. Charlo

Castaño faced unified light middleweight champion Jermell Charlo on 17 July 2021 in San Antonio, Texas in a showdown for the undisputed light middleweight championship. A competitive fight between them ended in a split draw, the second of Castaño's career, with scores of 117–111 Charlo, 114–113 Castaño and 114–114 even.^[16] The result was controversial, with much attention being brought to judge Nelson Vazquez's 117–111 Charlo card, which was described as "terrible" by Andre Ward.^[17] After the fight, an upset Castaño opined, "[Charlo] won the second, tenth and eleventh [rounds], that’s it... I don’t agree with [the result]. I felt I was robbed, and I demand a rematch."^[18]

Castaño vs. Charlo II

On November 27, 2021, it was revealed by ESPN that Castaño and Charlo had agreed to face each other in a rematch.^[19] The fight was officially announced on February 8, 2022, and was supposed to take place on March 19.^[20] Castaño was forced to withdraw on February 17 however, after suffering a slight biceps tear during training camp.^[21] A day later, the WBO ordered Castaño to show cause as to why Charlo should't be rescheduled to make an overdue mandatory title defense against Tim Tszyu.^[22] The fight was granted sanctioning approval by the WBO on May 3, under the condition that it takes place by May 14.^[23] The bout was officially announced for May 14, 2022, and took place at the Dignity Health Sports Park in Carson, California.^[24] He lost by 10th-round knockout after being knocked down twice.^[25] He was down on all three of the judges' scorecards at the time of the stoppage, with scores of 84–87, 83–88 and 82–89.^[26]