The South Korean technology giant inspected 105 Chinese partners in September following a report by China Labor Watch that at least one supplier was employing children under the age of 16.
The US-based watchdog said staff at some plants were forced to work up to five times the legal overtime limit and denied basic labour rights.
Samsung said its investigators had reviewed employment records at all 105 suppliers and conducted face-to-face ID checks, but "did not identify any instance of child labour".
However, they did find "several instances of inadequate practices" including excessive overtime and a system of fines imposed for lateness or absenteeism.
"We have identified the need for initiatives to reduce employee overtime as a top priority, and we are researching and developing measures that will eliminate hours beyond legal limits by the end of 2014," Samsung said in a statement.
Samsung said it would finish reviewing 144 additional suppliers by the end of this year.
From 2013, working conditions at all 249 suppliers in China would be monitored by a third party audit programme, it added, vowing to terminate contracts with any found to be using child labour.
All suppliers have been told to correct irregularities in labour contracts, and to provide adequate safety equipment and sufficient safety training, the statement said.
Managers will receive additional training on sexual harassment and physical and verbal abuse, while hotlines are being established for workers to report any inhumane treatment or labour violation, Samsung said.