James Stephens (Irish: Séamus Mac Stiofáin; 26 January 1825 – 29 March 1901) was an Irish Republican, and the founding member of an originally unnamed revolutionary organisation in Dublin. This organisation, founded on 17 March 1858, was later to become known as the Irish Republican Brotherhood (I.R.B).

References to his early life, according to one of his biographers, Desmond Ryan, are obscure and limited to Stephens' own vague autobiographical recollections. James Stephens was born at Lilac Cottage, Blackmill Street, Kilkenny, on 26 January 1825 and spent his childhood there. No birth records have ever been located, but a baptismal record from St. Mary's Parish is dated 29 July 1825. According to Marta Ramón, there is reason to believe that he was born out of wedlock in late July 1825; however, according to Stephens his exact date of birth was 26 January.

The son of John and Anne Stephens (née Casey), he had five brothers and sisters: Walter, John, Francis, who died when James was ten, Harriet, who had died by July 1848, and Anne, who died just after his flight into exile, as did his father. By 1856, Stephens' remaining family had vanished without a trace, according to Ramón.

For many years, his father had been a clerk to auctioneer and bookseller William Jackson Douglas whose offices and warehouses were on High Street, Kilkenny. Ryan has the order of the name as William Douglas Jackson, of Rose Inn Street, in his Stephens' biography Fenian Chief. John Stephens, as well as his earnings as a clerk, also had some small property in Kilkenny; records show him as occupier of 13 Evan's Lane, St. Mary's Parish and 22 Chapel Lane, St. Canice's Parish, his residence at the time of the rising.

Little is known of Stephens's mother and according to Ryan, it is possible he had no memory of her. Only briefly does Stephens mention her in his writings, although her name appears on Stephens' marriage certificate in 1863. His mother's people, the Caseys, were shopkeepers; one account says they ran a small hardware business. In April 1846 James and his sister, Anne, became sponsors to their cousin, Joseph Casey, at his baptism in St. Mary's Cathedral, Kilkenny. Joseph Casey would later be acquitted of charges of suspected Fenian activities in England in 1867. During Stephens' second exile in Paris, he would spend much time with the Caseys, who emigrated to France after the trial of Joseph. Stephens would be expelled from France on 12 March 1885 because of a series of press interviews given by Patrick Casey advocating the "dynamite war," which Stephens had repudiated consistently.

Because his father was intent on giving his son the best education his means would allow, Stephens was registered as a day pupil at St. Kieran's college for at least one quarter in 1838. Prior to this, he had attended St. Kieran's school beside his home, before the school moved to College Road.

An omnivorous reader, Stephens, according to Ryan, was a silent and aloof student with a thirst for knowledge, a characteristic throughout his life. Aged 20, Stephens was apprenticed to a civil engineer and obtained a post in a Kilkenny office for work then in progress on the Limerick and Waterford Railway in 1844.

Stephens was in a romantic relationship with a young lady, Miss Hilton, at this time, although she did not share his nationalist sentiments. This relationship ended shortly after the rising.