The Chiba Lotte Marines (千葉ロッテマリーンズ, Chiba Rotte Marīnzu) are a professional baseball team in the Pacific League based in Chiba City, Chiba Prefecture in the Kantō region of Japan and owned by Lotte Holdings Co., Ltd. The Marines were a founding member of the Pacific League in 1950 as the then-known Mainichi Orions when the Japanese Baseball League reorganized into Nippon Professional Baseball, where they won the inaugural 1950 Japan Series. Since 1992, the Marines' home ballpark has been the ZOZO Marine Stadium, located in the Mihama Ward of Chiba, with seats for up to 30,118 people.

The "Marines" name originates from the name of the ballpark, also officially named the Chiba Marine Stadium, because it is located at the seaside. Six players have won the Nippon Professional Baseball Most Valuable Player Award with the team; the last member to win the MVP was in 1985, the longest current drought for all twelve teams.

As of the end of the 2025 season, the franchise's all-time record is 4930-4928-408 (.500).

The franchise began in 1950 as the Mainichi Orions, an inaugural member of the Pacific League, and were owned by the Mainichi Shimbun newspaper. The Orions were named after the constellation of the same name. The Marines won the inaugural Japan Series in 1950.

In 1958, the team was merged with the Daiei Unions and renamed the Daimai Orions, to reflect that both Daiei and Mainichi had a 50% stake in the team, with control being given to Daiei Film president Masaichi Nagata. In 1964 they became the Tokyo Orions, and then the Lotte Orions in 1969. The franchise was slow to replicate its initial success: the Orions made the Japan Series in 1960 and 1970, only to lose both years.

The team played in central Tokyo until 1972. From 1973 to 1977 the Lotte Orions played in the northern Japanese city of Sendai. In 1974, they beat the Chunichi Dragons, becoming the first Pacific League team to win the Series in ten years, as the Yomiuri Giants had claimed the prior nine titles behind the Oh–Nagashima attack. After beating the Dragons, their owners, Lotte Holdings, decided to hold their victory parade in Tokyo, which surprised fans in Sendai. This eventually caused their attendance there to dwindle, going from sold-out games in 1973, to only about 2,000–3,000 attendees for their last few years in Sendai.

In 1977, the Orions signed Major League Baseball player Leron Lee, who ended up playing for the team for eleven seasons, compiling a .320 career batting average and slugging 283 home runs with 912 career RBI. From his retirement to early 2018 (when surpassed by Norichika Aoki), Lee held the Japanese record for career batting average (players with more than 4,000 at bats). In 1978, Lee invited younger brother Leon Lee to play in Japan, and the brothers formed a feared cleanup for the Orions for five seasons — in 1980, Leron had 33 home runs, 90 RBI, and a batting average of .358; while Leon slugged 41 home runs and drove in 116 runs, with a batting average of .340.

In 1978 the team returned to the Tokyo area, settling in the Kawasaki Stadium, at one time home to the Taiyo Whales (now Yokohama DeNA Baystars).