《约翰一书》（希腊语：ΙΩΑΝΝΟΥ Α΄）是《新约圣经》中的第23卷书，属于“大公书信”中最前写成的第四卷。

约翰一书与《约翰福音》在文体上颇相似，因此可能是来自于同一个作者，福音书作者约翰。例如，他在信的起头提及自己曾亲眼见过“生命之道……原与父同在、且显现与我们那永远的生命”，这与约翰福音的序言颇相似。

这本书的真确性受到穆拉托里残片及诸如伊里奈乌斯、波利卡普和帕皮亚斯等第二世纪的早期作家所支持。据优西比乌斯（约公元260-342年）所说，约翰一书的真确性从未受人质疑。

有些较古老的译本在约翰一书5：7的结尾部分和5：8的开头部分加上了以下一句话：“在天上……有父、道和圣神：这三位是合一的。在地上作见证的有三样。”（《圣经英王詹姆斯译本》）可是，这节经文从没有在任何早期的希腊文手抄本里出现过。因此这节经文可能是有些人为了支持三位一体的教义而杜撰的。大部分现代译本，无论是天主教抑或是基督新教出版的，均没有把这句话包括在圣经的正文中。

The First Epistle of John, often referred to as First John (US) or One John (UK) (and written 1 John) is a book of the New Testament. This fourth catholic or "general" epistle is attributed to John the Evangelist, traditionally thought to be the author of the Gospel of John and the other two Epistles of John. This Epistle was probably written in Ephesus between the years 95–110. The work was written to counter docetism, which is the belief that Jesus did not come "in the flesh", but only as a spirit. It also defined how Christians are to discern true teachers: by their ethics, their proclamation of Jesus in the flesh, and by their love.