在哲学中，道德相对主义是一种立场，认为道德或伦理并不反映客观或普遍的道德真理，而主张社会、文化、历史或个人境遇的相对主义。道德相对主义者与道德普遍主义相反，坚持不存在评价伦理道德的普遍标准。相对主义立场认为道德价值只适用于特定文化边界内，或个人选择的前后关系。极端的相对主义立场提议其他个人或团体的道德判断或行为没有任何意义。
一些道德相对主义者，例如存在主义者让-保罗·萨特坚持个人的、主观的“道德核心”（moral core）应该成为个体道德行为的基础。公共道德反映社会习俗，只有个人的、主观的道德表达真正的本真（Authenticity）。
一些演化生物学家认为道德相对主义是自然选择导致的结果。在这种情况下，道德被定义为一些能够促进物种生存和繁殖的社会实践。
道德相对主义与道德普遍主义（moral universalism）观点相对，如世界人权宣言就是道德普遍主义实践。同时道德相对主义与道德客观主义（moral objectivism）、道德现实主义（moral realism）和道德自然主义（moral naturalism）相区别。
Many people in other times and places thought slavery, for example, acceptable, even good; while most today view it as a great evil. Many writers and thinkers have held that one can justify any number of evils based on subjective or cultural preferences, and that morality requires some universal standard against which to measure ethical judgments. A moral relativist might respond that this criticism is only valid if one already accepts that such acts are indeed fundamentally evil - a position which the moral relativist would deny - and that the objection is therefore nothing other than an uncritical statement that morals are in fact absolute.
However, if it is society that has, for instance, rejected slavery, they presumably agree that it is wrong, if their position is one of social or cultural moral relativism. This presents a meta-ethical problem in explaining what happens when a society has a collective change of heart. Consider the case of someone who has minority moral views within their society, and yet is vindicated (even relativistically) by future developments. If "right" and "wrong" literally mean "what my society accepts/rejects" then a social moral relativist in a slave-owning society of the past who says "slavery is wrong" is effectively saying "slavery is not approved of by my society", which is false — false. Yet, the relativist of the present is committed to agreeing with the relativist of the past, since they both oppose slavery.
The argument was phrased in terms of cultural relativism, but a similar argument applies to subjectivism. It is difficult for a moral subjectivist to claim that they have undergone any personal moral improvement, or that an attitude they used hold was wrong, when it was obviously what they felt was right at the time. For them, there is no external standard to judge against, so while their attitudes change, they cannot be said to improve or decline. It therefore seems that there is a difference in what can be expressed or justified between an objectivist and a relativist, although whether it involves the loss of anything worthwhile is open to debate.
早期历史阶段的人类社会曾经接受，甚至推崇奴隶制度，但现在若仍持如是观点，则冒天下之大不韪。很多作家和思想家相信，在特定的主观偏好或者文化习惯下，不论何种恶行，都可能自圆其说；道德的确立则要依托于普世价值，唯其如此，方能做出道德维度的审判。道德相对主义者或许会承认这样的论调，但要让人接受有些恶行在根本上就是恶的，则无法接受；他们认为，这样的批评本质上仅仅是对道德的绝对性的随声附和。
而倘若是“他们”的社会，比方说，拒绝了奴隶制度，则想必多数人认为奴隶制是错的，如果这个社会承认社会、文化、和道德的相对性。要解释一个社会集体性的态度转变，就是元伦理学层面的问题。试想，一个社会里持有少数派观点的人，可能会被后来未来发展后的社会平反（即使也是相对意义上的）。倘若“对”与“错”仅仅在字面意义上代表“我身处的社会接受/反对什么”，那一个生活在历史上奴隶社会的道德相对主义者若说“奴隶制是错的”，则实际上是在说“奴隶制在我身处的社会不被允许”---这样的结论却是错的，错在违背了历史事实。然而一个生活在当代社会的的相对主义者则完全同意曾经的相对主义者，因为他们都反对奴隶制。
虽说该论点以文化道德主义的语言陈述，但相似的论点同样适用于主观主义。对于一个道德主观主义者，很难说他们经历过任何道德上的进步；在特定的历史阶段的允许下，也很难说他们的哪种态度和感受是错的。对他们而言，由于缺乏外在的判断标准，即便是他们对于某一问题的态度有所转变，也不能说他们是进步了还是退步了。因此，客观主义者和主观主义者对于什么可以被表达，什么可以自圆其说，貌似存在着分歧---至于是否有任何有价值的地方被二者忽视，则尚可讨论。
There are also difficulties in putting a boundary upon "society" or "culture" - what people feel to be their social or cultural groupings may well not align with legal and national groupings. The person holding "minority moral views within their society" may consider their "culture" more aligned with that minority than with the larger state or national society which determines what is lawfully acceptable,. This can be seen, for instance, where religious communities within a nation or state hold views on the morality of issues such as abortion or homosexuality which differ from the current legal position on those issues. This flexibility could lead to the objection that cultural moral relativism is "anything goes", since one could find — or found — a society that condones whatever one wishes to do.
The equivalent of such gerrymandering in subjective or personal moral relativism would be for an individual to adopt different principles at different times, which would lead to a very acute form of "anything goes", unless forestalled by a meta-ethical principles that individuals need to be self-consistent. It could be argued that jumping ship in this way is dishonest, but the ethical acceptability of honesty is presumably as relative as anything else if relativism is true.
元伦理学（英语：meta-ethics）是伦理学的一个分支，以伦理学本身作为其研究对象。元伦理学起源于20世纪初，乔治·爱德华·摩尔于1903年发表的《伦理学原理》宣告了元伦理学的诞生。之后，元伦理学取代了实践伦理学和规范伦理学成为了西方伦理学的主流，直至1970年代为止。
现代元伦理学有各种不同的流派。包括以乔治·爱德华·摩尔、哈罗德·亚瑟·普里查德（Harold Arthur Prichard）、W·D·罗斯（W. D. Ross）等为代表的直觉主义；以伯特兰·罗素、路德维希·维特根斯坦、维也纳学派等为代表的情感主义；还有以斯蒂芬·图尔敏（Stephen Toulmin）、R·M·黑尔（R. M. Hare）等为代表的规定主义。
Some philosophers, for example R. M. Hare (1919 – 2002), argue that moral propositions remain subject to logical rules, notwithstanding the absence of any factual content, including those subject to cultural or religious standards or norms. Thus, for example, they contend that one cannot hold contradictory ethical judgments. This allows for moral discourse with shared standards, notwithstanding the descriptive properties or truth conditions of moral terms. They do not affirm or deny that moral facts exist, only that logic applies to our moral assertions; consequently, they postulate an objective and preferred standard of moral justification, albeit in a very limited sense. Nevertheless, according to Hare, logic shows the error of relativism in one very important sense (see Hare's ). Hare and other philosophers also point out that, aside from logical constraints, all systems treat certain moral terms alike in an evaluative sense. This parallels our treatment of other terms such as or , which meet with universal understanding and do not depend upon independent standards (for example, one can convert measurements). It applies to good and bad when used in their non-moral sense, too; for example, when we say, "this is a wrench" or "this is a wheel". This evaluative property of certain terms also allows people of different beliefs to have meaningful discussions on moral questions, even though they may disagree about certain "facts".
一些人将卡尔·马克思（1818 – 1883）作为道德相对主义的典型。他宣称每个社会的道德体系完全是其生产方式和阶级结构的产物。他相信统治阶级的利益将成为社会的占优势的道德体系。
弗里德里希·尼采认为道德是二元论概念带给人们、又由教会继续维持的一个错误思想。他一生的任务，就是重新评估所有价值，把人类思想从这些错误拯救出来。他预想未来个体将自然地行动，充分利用他们的天赋潜力或权利欲（der Wille zur Macht）。他相信人类只有通过开始自然地、本能地、根据每个个人的愿望和动力行动，才会前进，实现这潜力。Übermensch将呈现强壮、有力、自然和快乐的结果。快乐（“力量增长、阻力被克服的感觉”）将自然出现。尼采希望通过人类动力“再自然化”来准备人类成长的土壤。他相信，一旦人们摆脱道德（以及协会、宗教），下一代人将会不受污染地、自由地、强壮地成长。（见《善恶的彼岸》、《偶像的黄昏》、《敌基督》等）。
道德相对主义不可避免地反对几乎世界所有宗教所教导的绝对道德。
佛教在基本教义三法印中提出诸行无常的概念，意思是指，这个世界上的一切存在，其存在的根据皆是依赖于其他的因素。因此，所有的一切皆是无常、生灭的，世界上并不具有一个独立自存而永恒、不变的实体。于如此立场下，道德法则也可被理解为依赖于其他因素而存在，并因此不具备绝对的本质、价值。
功利主义 · 归结主义 · 义务论 · 关怀伦理学 · 德性伦理学 · 情绪主义 · 道德相对主义 · 道德普遍主义
柏拉图 · 亚里士多德 · 孔子 · 孟子 · 荀子 · 奥古斯丁 · 阿奎那 · 斯宾诺莎 · 休谟 · 康德 · 黑格尔 · 叔本华 · 边沁 · 密尔 · 齐克果 · 西季威克 · 尼采 · 摩尔 · 罗尔斯 · 威廉姆斯 · 麦基 · 麦金泰尔 · 辛格 · 帕菲特 · 斯坎伦 · 内格尔 · 泰勒 · 阿苏尔门迪（英语：Joxe Azurmendi）
伦理困境 · 演化伦理学 · 伦理学历史 · 法律哲学 · 社会哲学 · 政治哲学
