Opening Day is the day on which professional baseball leagues begin their regular season. For Major League Baseball (MLB) and most of the American minor leagues, this day typically falls during the first week of April, although in recent years it has occasionally fallen in the last week of March. As of 2025, Opening Day falls on the last Thursday of March or the first Thursday of April. In Nippon Professional Baseball, this day typically falls during the last week of March.

For baseball fans, Opening Day serves as a symbol of rebirth; writer Thomas Boswell once penned a book titled, Why Time Begins on Opening Day. Pre-season exhibition games are usually played in the month before Opening Day, during spring training. A home opener is a team's first game of the season on their home field.

Equivalents to Opening Day occur throughout the sport, including minor leagues, college baseball, high school, and youth leagues. Because MLB generally begins its season earlier than the other professional baseball leagues, its Opening Day is the one most commonly recognized by the general public. Most minor leagues start a few days later, but within the same week; the short season Class A and Rookie leagues are exceptions, as they begin in June. College, high school and youth baseball seasons vary widely depending on location and weather conditions.

For generations, Opening Day has arrived amid pageantry. In Cincinnati, home of the sport's first openly all-professional team, the annual Findlay Market Parade marks an official "city holiday" with young and old alike taking the day off to cheer on the Reds. For decades, the first pitch of every major league season officially took place in Cincinnati, and the Reds remain the only major league team scheduled to always open the season with a home game (the sole exceptions, since the beginning of the 20th century, being in 1966, when they started the season at Philadelphia after rain washed out the opening series in Cincinnati; 1990, when due to a lockout affecting the schedule they opened the season at Houston; and 2022, when another lockout led to their opening the season at Atlanta). The Chicago Cubs have been the Reds' most frequent Opening Day opponent, visiting Cincinnati for 36 season openers, most recently in 2007. The Pittsburgh Pirates, against whom the current Reds organization played their first opener in 1882, are a close second with 32, most recently in 2023; no other team has more than 19 (by the St. Louis Cardinals, most recently in 2014), largely due to the Cubs and Pirates rotating as the Opening Day opponents from 1899 to 1916, then the two teams and the Cardinals rotated from 1917 to 1952. Following the then-Boston Braves relocation to Milwaukee during the 1953 spring training, the Braves swapped schedules with the Pirates and the Opening Day opponent for the Reds began to be rotated amongst the rest of the National League. Fittingly, the Reds were also the first team to host an Interleague game on Opening Day when the team hosted the Los Angeles Angels of Anaheim in the first year of year-round Interleague play in 2013.

Since 1994 ESPN has often televised a regular-season game the night before "Opening Day" and recent years have seen the staging of season-opening series in Mexico, Puerto Rico, Japan, and Australia. While these are technically "opening games", Major League Baseball still reserves the title "Opening Day" for the first day in which multiple games are played. (For the first time ever, three televised games were played on Sunday, April 3, 2016, before the traditional "Opening Day" slate of games on Monday, April 4.)

Opening Day is a state of mind as well, with countless baseball fans known to recognize this unofficial holiday as a good reason to call in sick at work or be truant from school (as most teams typically play their home opener on a weekday afternoon) and go out to the ballpark for the first of 162 regular season games. Teams' home openers serve as the only regular season games during the year in which the entire rosters of both teams as well as coaches and clubhouse staff are introduced to the crowd prior to the games; for the rest of the year, ballparks only introduce the starting lineups and the teams' managers. Some teams, among them the New York Mets, have had their broadcasters as the master of pre-game ceremonies for their home openers, which also typically feature appearances by retired players, local celebrities or media personalities, politicians, and other dignitaries.

Prior to Opening Day, the teams' managers have to decide the starting pitchers for the game, an assignment typically given to the ace of each team's staff. For a pitcher to start on Opening Day is considered an honor, regardless of whether they are on the home or visiting team. Hall of Fame pitcher Early Wynn, who played for the Washington Senators, Cleveland Indians and Chicago White Sox, once said: "An opener is not like any other game. There's that little extra excitement, a faster beating of the heart. You have that anxiety to get off to a good start, for yourself and for the team. You know that when you win the first one, you can't lose 'em all."

In 2014, Ozzie Smith, with the support of Anheuser-Busch, began a campaign using the We the People site on WhiteHouse.gov to petition the U.S. government to make Opening Day an official national holiday.