The incident occurred at around 8pm at a home in Büsserach, a town of about 2,000 people, after a dispute that led to the attacker pulling out a hunting rifle to shoot his 55-year-old brother, cantonal police said.
The victim was seriously wounded in the incident that involved a dispute between the two men and another brother, police said.
The wounded man was transported by the Rega emergency rescue service to hospital, where he died from his injuries overnight.
The gunman was arrested without resistance at the crime scene and confessed to having shot his brother, police said.
No-one else was injured in the altercation.
According to an online report from 20 Minuten, the disagreement arose over an inheritance dispute and the conditions under which the 48-year-old brother lived in the home.
His two brothers wanted him to start paying a nominal rent after living there rent-free, the newspapers said, citing the third brother.
“I panicked and was afraid he would shoot at me,” the uninjured brother said.
“I took flight.”
The shooter had lived in the house since he was a child, while the victim was living elsewhere in Büsserach with his girlfriend, 20 Minuten said.
The brothers’ father, a farmer, died three years ago, while their mother died two years ago, which gave rise to disagreements over the legacy, a neighbour told the newspaper.
The self-acknowledged killer was unemployed and was regarded as a “black sheep” in the family, the surviving brother said.
Police and the canton of Solothurn’s prosecutor’s office have launched an investigation into the case.