天干，是中国古代的一种文本计序符号，共10个字: 甲、乙、丙、丁、戊、己、庚、辛、壬、癸，循环使用。

中国等汉字文化圈国家古代常以之来命名、排序、纪时。

天干最晚在夏朝已经开始，明显的证据是夏朝后期有帝王叫做孔甲、胤甲。但是正式将这十个符号命名为天干应该与地支有关：干为主干，支为分支，两个相对的命名应当是一起出现的。在商朝开始出现天干与地支配合用以纪日，使用干支纪年要更晚一些。

关于天干何来说法众多，其中一种是古人根据事物十个状态排序以定的，视为循环。其中植物的生长就是一例︰按

现代研究的结论之一是天干源于原始人类以人体部位计数、排序，下面列出《说文解字》中的释义：

The ten Celestial or Heavenly Stems (Chinese:天干; pinyin:tiāngān) are a Chinese system of ordinals that first appear during the Shang dynasty, ca. 1250 BCE, as the names of the ten days of the week. They were also used in Shang-period ritual as names for dead family members, who were offered sacrifices on the corresponding day of the Shang week. The Heavenly Stems were used in combination with the Earthly Branches, a similar cycle of twelve days, to produce a compound cycle of sixty days. Subsequently, the Heavenly Stems lost their original function as names for days of the week and dead kin, and acquired many other uses, the most prominent and long lasting of which was their use together with the Earthly Branches as a 60-year calendrical cycle.