Jonathan Gerardo Castellanos Flores
- Bats Right, Throws Right
- Height 6' 0", Weight 215 lb.
- High School Josefa Ortiz de Dominguez High School
Jonathan Castellanos has pitched pro ball for over 20 years, peaking at AAA.
Castellanos debuted in 1999 with AZL Mexico, going 5-3 with a 4.58 ERA to tie fellow Monterrey native Juan Jesus Martinez for the team lead in wins. He tied for 8th in the Arizona League in victories. Signing with the Arizona Diamondbacks (the scout was Mike Rizzo) , he was 2-4 with a 3.45 ERA for the 2000 DSL Diamondbacks.  He split 2001 between the South Bend Silver Hawks (1-3, 4.86) and Yakima Bears (2-3, 4.08). He allowed a Northwest League-high 100 hits but was 4th in IP (86) and tied Geoff Jones for 9th in Ks (73).
In 2002, he was 4-10, 4.36 for South Bend. He tied for 4th in the Diamondbacks chain in losses but three of the four hurlers he tied with would pitch in the majors at some point - Jason Bulger, Eric Knott and Beltran Perez. After going 7-3 with a 4.63 ERA for the 2003 Lancaster JetHawks. He returned to Mexico for a year and was 0-3 with a 4.81 ERA in 2004, his first season for his hometown Monterrey Sultans. 
The right-hander was active in the US again in 2005, with a 10-3, 4.65 record for the JetHawks. He tied James Holcomb and Jason Mackintosh for 7th in the California League lead in wins and came close to the top ten in ERA. On the other hand, his 24 homers allowed tied Mike Prochaska for the lead.  He tied Steven Jackson for 6th in the Arizona chain in wins and tied Edgar González for 8th with 116 whiffs. In 2006, he was with the Tennessee Smokies (3-1, 4.50 in 24 G) and Tucson Sidewinders (1 R in 2 IP).
That winter, he was 2-2 with a 5.49 ERA for the Naranjeros de Hermosillo. He split 2007 between the Mobile BayBears (5-6, 3.82) and Monterrey (2-1, 1.80), ending his time in the D'backs chain. He was 2-2 with a 3.54 ERA for Hermosillo in 2007-2008. He had a 6-9, 4.03 record for the 2008 Sultans, walking only 20 in 73 2/3 IP. He tied for 9th in the Mexican League in losses. He was 3-3 with a 2.98 ERA in winter ball for Hermosillo.
Castellanos began his second decade in pro ball by having a 4-4, 3.84 campaign for Monterrey then was lit up in the winter (0-1, 10.13, 12 H in 5 1/3 IP). He recovered in the summer to go 6-2 with a 2.90 ERA. Had he qualified for the LMB ERA lead, he would have been second to Mac Suzuki by .01.  He was 3-2 with a 4.34 ERA for the 2010-2011 Naranjeros. He fell to 4-4, 5.84 for the 2011 Sultanes and was 4-5 with a 4.11 ERA for Hermosillo in 2011-2012, tying for 6th in the Mexican Pacific League in losses.
The veteran was 10-6 with a 4.10 ERA for his hometown team in 2012. He tied for 10th in the LMB in wins. He followed with a 3-1, 3.67 winter, working mostly out of the bullpen. In '13, he was 10-8 with a 4.91 ERA for Monterrey, walking only 27 in 117 1/3 IP. He tied Luis Mendez and Fredy Quintero for 6th in the circuit in wins. He moved that winter to the Cañeros de Los Mochis and was 4-1 with a 3.38 ERA.
After eight seasons with Monterrey, he moved to the Leones de Yucatán in 2014, going 10-9 with a 3.06 ERA. He tied for 8th in the league in wins and was 6th in ERA (between Baudel Zambrano and Ramon Garcia). He was 4-5 with a 3.89 ERA for Los Mochis in 2014-2015, tying for 7th in losses. With the 2015 Leones, he had a 7-7, 3.90 season. He did not pitch in the winter for the first time in 8 years.
For the 2016 Leones, he was 11-3 with a 2.89 ERA. He was 8th in ERA (between veteran star Francisco Campos and Angel Araiza) and tied Alex Delgado for 5th in wins.  He was 7-5 with a 4.73 ERA for Yucatán in 2017, then returned to winter competition with the Cañeros and was uncharacteristically wild (37 BB in 68 IP); he was 2-4 with a 4.24 ERA.
The Mexican League went to a two-season format for 2018. He was 5-3 with a 2.43 ERA in the spring season. In the fall campaign, he pitched for the Leones (1-1, 5.56) and the Tecolotes de los Dos Laredos (0-3, 6.89). He was 6-1 with a 4.61 ERA for Los Mochis in 2018-2019, tying for the most wins in the LMP with Andrés Ávila, Dennis O'Grady, Konner Wade and Elian Leyva.
Despite having been a reliever in the 2018-2019 LMP, he made the Mexican national team as a starter for the 2019 Pan American Games Qualifier. While the team fell short of a spot in the 2019 Pan American Games, he was the only player in the tournament to beat the Dominican national team. He went 5 1/3 IP (5 H, 0 BB, 3 K, 2 R, 2 ER); Óscar Félix relieved in what wound up a 3-2 win over Carlos Pimentel. 
His third decade in pro ball began with the 2019 Saraperos de Saltillo.