Jus sanguinis (English: /dʒʌs ˈsæŋɡwɪnɪs/ juss SANG-gwin-iss or /juːs -/ yooss -⁠, Latin: [juːs ˈsaŋɡwɪnɪs]), meaning 'right of blood', is a principle of nationality law by which nationality is determined or acquired by the nationality of one or both parents. Children at birth may be nationals of a particular state if either or both of their parents have nationality of that state. It may also apply to national identities of ethnic, cultural, or other origins. Citizenship can also apply to children whose parents belong to a diaspora and were not themselves citizens of the state conferring citizenship.^{[citation needed]} This principle contrasts with jus soli ('right of soil'), which is solely based on the place of birth.

In the 21st century, almost all states apply some combination of jus soli and jus sanguinis in their nationality laws to varying degrees, in contrast to largely pure forms of either as used in the 19th and 20th centuries. Historically, the most common application of jus sanguinis is a right of a child to their father's nationality. Today, the majority of countries extend this right on an equal basis to the mother. Some apply this right irrespective of the place of birth, while others may limit it to those born in the state. Some countries provide that a child acquires the nationality of the mother if the father is unknown or stateless, and some irrespective of the place of birth. Some such children may acquire the nationality automatically while others may need to apply for a parent's nationality.

Most countries with mixed systems extend jus soli citizenship on a limited basis only to children born in their sovereign territory who do not qualify for citizenship elsewhere to avoid statelessness—an obligation for states party to the Convention on the Reduction of Statelessness.

When the sovereignty over a territory is transferred from one state to another, individuals may be given the right of option of nationality based on jus sanguinis, typically on the condition of emigration from the ceded territory, or other rules.

Originally, the term nation was synonymous with ethnicity. However, at the end of the 19th century, the French-German debate on nationality saw the French, such as Ernest Renan, oppose the German conception, exemplified by Johann Fichte, who believed in an 'objective nationality', based on blood, race, or language. Renan's republican conception (with perhaps the presence of a German-speaking population in Alsace-Lorraine) explain France's early adoption of jus soli.

Many nations have a mixture of jus sanguinis and jus soli. For example, the United States grants citizenship based on jus soli to almost all people born within its borders, and also grants citizenship based on jus sanguinis to children born outside its borders to U.S. citizen parents subject to the parents meeting certain residency or physical presence criteria. The United Kingdom usually applies jus sanguinis, but also has a jus soli principle for children of foreign citizens living in the U.K. depending on the parents' nationality and legal residence status.

About 60% of all countries worldwide have a limited jus soli principle that extends citizenship to children born within their borders who do not qualify for citizenship in any other country. Countries which ratified the Convention on the Reduction of Statelessness are obligated to enact laws as a solution for statelessness.

Some modern European states which arose out of the dissolved Austro-Hungarian or Ottoman empires have huge numbers of ethnic populations outside of their new 'national' boundaries, as do most of the former Soviet states. Such long-standing diasporas do not conform to codified 20th-century European rules of citizenship.