The number of recorded incidents of self-harm at HMP Moorland has risen from 451 in 2016-17 to 543 in 2017-18 and 755 in 2018-19, its independent monitoring board (IMB) said.
The number of assaults on staff has also leapt from 66 in 2016-17 to 119 in 2018-19, according to the IMB's annual report.
The category C male resettlement prison near Hatfield Woodhouse in South Yorkshire - which also acts as a young offender institution - holds around 1,000 inmates.
Prisoners can phone the Samaritans helpline from their cell but calls are cut off after 15 minutes and the line is not available after 11pm.
Mental health staff now cover the prison seven days a week, the report added.
The IMB said: "The big increase in incidents of self-harm is a cause for concern, but it is hard to know whether this may, in part, result from better monitoring and recording.
"Violence against prisoners and staff peaked in the summer of 2018, but there appears to have been a downward trend since then, which tends to confirm the IMB's sense that the prison is becoming calmer and safer."
The prison's IMB chairman, Esther Beeston, said: "The prisoners have benefited from the employment of additional officers, extra staff training and improved security measures.
“Over the last year, there have been improvements in many areas and the atmosphere throughout the prison has become calmer."
IMBs are made up of volunteers appointed by justice ministers to scrutinise prison conditions.