"Local law enforcement authorities have publicly advised that the available evidence does not permit them to determine who caused the child's injuries. Similarly, based on the evidence presently available, the NFL cannot conclude that Mr. Hill violated the Personal Conduct Policy," the league said in a statement.
Hill is clear to attend Chiefs training camp - which begins July 26 - and participate in all club activities.
The league said that its "primary concern has been the well-being of the child" during the four-month investigation. The NFL statement added that the 3-year-old son of Hill and former fiancee Crystal Espinal is "safe" and noted that the Johnson County Department for Children and Families remains involved.
The Chiefs confirmed Hill will attend the start of training camp.
"Based on the information provided to us by the league, we have decided it is appropriate for Tyreek to return to the team at the start of training camp," the team said in a statement. "The club fully supports the conditions for return laid out by the league and will continue to monitor any new developments in the case."
Hill was barred from all team-related activities after an audio clip of the receiver and Espinal discussing their son's injuries was broadcast by KCTV5 on April 25.
In the recording, Hill denied breaking his son's arm. In response to Espinal saying their son fears him, Hill said she should be "terrified" of him.
The NFL explained in an email to Pro Football Talk's Mike Florio that the comment didn't justify a suspension or fine.
"When viewed in the context of the full 11-minute, 27-second audio recording and all other information gathered, the statement did not rise to a level of warranting discipline under the personal conduct policy," wrote NFL spokesman Brian McCarthy.
The recording was released a day after Johnson County District Attorney Stephen Howe announced he wouldn't be filing charges against Hill and Espinal. He said he believed a crime had occurred but that there wasn't evidence to conclusively prove who committed it.
In an extended version of the 11-minute conversation that was released last week, the wideout appeared to deny the 2014 assault of Espinal that he pleaded guilty to prior to entering the NFL.
The NFL had possessed the full audio of the conversation since April, sources told Albert Breer of MMQB.
The league reportedly made multiple, unsuccessful attempts to interview Espinal, Breer added.
The Chiefs and Hill could reportedly re-engage in talks about a contract extension, though it could be "several weeks if not months" before they resume negotiations, and any new deal could have "very specific language," according to James Palmer of NFL Network.
Hill has been a key piece of the Chiefs' offense since being selected in the fifth round of the 2016 draft. Last year, he helped get Kansas City to the AFC Championship Game with a 1,479-yard, 12-touchdown campaign that earned him first-team All-Pro honors.