| ２０２０年１２月 | |
|---|---|
| １ | 熊代聖人が国内ＦＡ権を行使して残留 |
| ２ | ショーン・ノリンが退団 |
| ３ | |
| ４ | 増田達至が国内ＦＡ権を行使して残留 |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | |
| １３ | |
| １４ | |
| １５ | |
| １６ | 源田壮亮が遊撃手部門でベストナインを受賞 栗山巧が指名打者部門でベストナインを受賞 |
| １７ | 平良海馬が最優秀新人賞を受賞 多和田真三郎と育成選手契約を締結 |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | |
| ２２ | |
| ２３ | |
| ２４ | リード・ギャレットと２０２１シーズンの契約を締結 |
| ２５ | ２０２１シーズンのチームスローガンが「ＢＲＥＡＫ ＩＴ」に決定 |
| ２６ | |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ３１ | |
| ２０２０年１１月 | |
| １ | 許銘傑コーチ、高木浩之コーチが退団 |
| ２ | |
| ３ | 多和田真三郎、相内誠、藤田航生に戦力外通告 |
| ４ | |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | 辻発彦監督の続投が決定 増田達至が最多セーブのタイトルを獲得 |
| １０ | |
| １１ | 高山久、鬼崎裕司、大石達也とコーチ契約を締結 |
| １２ | |
| １３ | 野田昇吾、國場翼、永江恭平、水口大地、森越祐人に戦力外通告 |
| １４ | |
| １５ | |
| １６ | |
| １７ | |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | 吉川光夫を北海道日本ハムファイターズから金銭トレードにより獲得 |
| ２２ | |
| ２３ | |
| ２４ | |
| ２５ | |
| ２６ | |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ２０２０年１０月 | |
| １ | |
| ２ | |
| ３ | |
| ４ | |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | |
| １３ | 外崎修汰が通算１００盗塁を達成 |
| １４ | |
| １５ | |
| １６ | |
| １７ | |
| １８ | |
| １９ | |
| ２０ | 高橋朋己が現役引退を発表 |
| ２１ | |
| ２２ | |
| ２３ | |
| ２４ | |
| ２５ | |
| ２６ | ドラフト会議において、渡部健人、佐々木健、山村崇嘉、若林楽人、大曲錬、タイシンガーブランドン大河、仲三河優太、赤上優人、長谷川信哉、宮本ジョセフ拳、豆田泰志、水上由伸の交渉権を獲得 |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ３１ | 平尾博司コーチとの契約を解除 |
| ２０２０年９月 | |
| １ | |
| ２ | |
| ３ | |
| ４ | |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | 山川穂高が通算１５０本塁打を達成 |
| １３ | |
| １４ | |
| １５ | 山川穂高が８月度のスカパー！サヨナラ賞を受賞 |
| １６ | |
| １７ | |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | |
| ２２ | 金子侑司が通算２００盗塁を達成 |
| ２３ | 西武として通算３０００勝を達成 |
| ２４ | |
| ２５ | |
| ２６ | |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ２０２０年８月 | |
| １ | |
| ２ | |
| ３ | |
| ４ | |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | |
| １３ | |
| １４ | |
| １５ | |
| １６ | |
| １７ | |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | |
| ２２ | 内海哲也が通算１５００奪三振を達成 |
| ２３ | |
| ２４ | |
| ２５ | |
| ２６ | 中村剛也が通算１５００安打を達成 |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ３１ | |
| ２０２０年７月 | |
| １ | |
| ２ | |
| ３ | |
| ４ | |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | |
| １３ | |
| １４ | |
| １５ | |
| １６ | 中村剛也が通算３００二塁打を達成 |
| １７ | |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | |
| ２２ | |
| ２３ | |
| ２４ | |
| ２５ | |
| ２６ | 栗山巧が通算３５０二塁打を達成 |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ３１ | |
| ２０２０年６月 | |
| １ | |
| ２ | |
| ３ | |
| ４ | |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | |
| １３ | |
| １４ | |
| １５ | |
| １６ | |
| １７ | |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | |
| ２２ | |
| ２３ | |
| ２４ | |
| ２５ | |
| ２６ | |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ２０２０年５月 | |
| １ | |
| ２ | |
| ３ | |
| ４ | |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | |
| １３ | |
| １４ | |
| １５ | |
| １６ | |
| １７ | |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | |
| ２２ | |
| ２３ | |
| ２４ | |
| ２５ | |
| ２６ | |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ３１ | |
| ２０２０年４月 | |
| １ | |
| ２ | |
| ３ | |
| ４ | |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | |
| １３ | |
| １４ | |
| １５ | |
| １６ | |
| １７ | |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | |
| ２２ | |
| ２３ | |
| ２４ | |
| ２５ | |
| ２６ | |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ２０２０年３月 | |
| １ | |
| ２ | |
| ３ | |
| ４ | |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | |
| １３ | |
| １４ | |
| １５ | |
| １６ | |
| １７ | |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | |
| ２２ | |
| ２３ | |
| ２４ | |
| ２５ | |
| ２６ | |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ３１ | |
| ２０２０年２月 | |
| １ | |
| ２ | |
| ３ | |
| ４ | |
| ５ | |
| ６ | |
| ７ | |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | |
| １３ | |
| １４ | |
| １５ | |
| １６ | |
| １７ | |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | |
| ２２ | |
| ２３ | |
| ２４ | |
| ２５ | |
| ２６ | |
| ２７ | |
| ２８ | |
| ２９ | |
| ２０２０年１月 | |
| １ | |
| ２ | |
| ３ | |
| ４ | |
| ５ | |
| ６ | |
| ７ | 秋山翔吾が海外ＦＡ権を行使してシンシナティ・レッズと契約を締結 |
| ８ | |
| ９ | |
| １０ | |
| １１ | |
| １２ | |
| １３ | |
| １４ | |
| １５ | |
| １６ | |
| １７ | |
| １８ | |
| １９ | |
| ２０ | |
| ２１ | |
| ２２ | |
| ２３ | |
| ２４ | |
| ２５ | |
| ２６ | |
| ２７ | |
| ２８ | |
| ２９ | |
| ３０ | |
| ３１ | |