December 30, 2018: Traded left-handed reliever Robby Scott to the Arizona Diamondbacks for cash. Scott was claimed off waivers on December 10, but was then designated for assignment on December 21, following the trade with the Dodgers.
January 21, 2019: Traded minor league second baseman Shed Long and a competitive balance draft pick in the 2019 MLB Draft to the New York Yankees for starting pitcher Sonny Gray and minor league left-handed pitcher Reiver Sanmartin. Long was then traded to the Seattle Mariners. Gray signed a three-year, $30.5 million contract extension through 2022, with a $12 million club option for 2023.
January 10, 2019: Signed backup catcher Curt Casali to a one-year, $950,000 contract, avoiding arbitration.
May 3: Derek Dietrich became the first major league second baseman to hit a pair of three-run homers in the first three innings of a game since Reds Hall of Famer Joe Morgan on Aug. 19, 1974, according to STATS.
May 28: Derek Dietrich hit three home runs and had 6 RBIs in a win over the Pittsburgh Pirates. The three home runs gave Dietrich 7 home runs versus the Pirates this season and also marked his last six hits all being home runs.
June 21: Derek Dietrich became the first player in the modern era (since 1900) to be hit three times by a pitch in one game according to the Elias Sports Bureau.
July 9: Luis Castillo pitched a scoreless fourth inning with two strikeouts for the National League, who lost to the American League 4–3 in the All-Star Game.
July 13: Against the Colorado Rockies, the Reds, according to Elias Sports, became the first Major League team in the modern era to ever collect five triples and at least three home runs in one game.
August 12: Aristides Aquino hit his 8th home run becoming the first player to ever hit eight home runs in his first 12 career games.
August 17: Aristides Aquino hit is 11th home run becoming the first player since 1900 to have 11 home runs in his first 17 games, according to the Elias Sport Bureau.
August 18: Kevin Gausman threw an immaculate inning, striking out all three batters on nine pitches, the 6th such feat in Reds history. This was the second time Gausman had thrown an immaculate inning, with the first coming on April 23, 2018.
August 28: Aristides Aquino hit his 13th home run of his career becoming the first player in the modern era to hit 13 home runs in his first 100 career plate appearances.
August 29: Aristides Aquino hit his 14th home run in August, breaking the National League rookie record for home runs in a month set by Cody Bellinger in June 2017.
September 2: Aristides Aquino hit his 15th career home run, becoming the fastest player to hit that mark in MLB history. Aqunio needed just 122 plate appearances, 13 fewer than the record set by Rhys Hoskins in 2017.
September 4: Michael Lorenzen became the second player in baseball history to hit a homer, earn the win as the pitcher and play in the field in the same game when the Reds defeated the Phillies, 8–5. The other player to achieve the feat was Babe Ruth on June 13, 1921.
September 4: The four home runs hit in the Reds 8–5 victory over the Phillies were by players who weren't in the starting lineup: Logan Morrison and Jay Bruce for the Phillies and Michael Lorenzen and José Iglesias for the Reds. It was the first time since 1900 in which four players who weren't in the starting lineup for either team entered the game and hit a home run, according to the Elias Sports Bureau.
September 18: Eugenio Suárez hit his 48th home run of the season, the most ever by a Venezuelan player, breaking the previous mark of 47 set by Andrés Galarraga in 1996.
September 25: Eugenio Suárez hit his 49th home run of the season, which moved him ahead of Mike Schmidt and Adrián Beltré for the modern NL record by a third baseman.
September 26: Marty Brennaman broadcasts his final game as the voice of the Reds on radio network 700 WLW. Brennaman has been the voice of the Reds for the past 46 seasons and is just one of eight major league broadcasters to cover one team for 46 seasons.
September 30: Eugenio Suárez is named the National League Player of the Month for the month of September. Suárez recorded a .337/.455/.747 slash line with 10 homers and 18 RBIs over 25 games in September. He is the second straight Reds player to win Player of the Month honors after Aristides Aquino was Player of the Month in August.