The 24-year-old will not be available for the crunch clash with Portsmouth on Saturday, but is set to join up with the squad next week ahead of the visit of Ipswich Town.
The Swansea City wing-back becomes Parkinson’s fourth addition of the window, with Antoine Semenyo set to become the fifth before the deadline.
“I’m pleased to welcome Declan to the club,” Parkinson said.
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“He’s another good addition to the squad and provides added depth in an area of the pitch we have been working to strengthen.
“Unfortunately he’s unavailable this weekend, but we look forward to him joining the squad next week.”
John said he believes the current 3-4-3 system will suit him ‘down the ground’.
“I’m looking forward to working under the manager and I believe it’s a system that will suit me down to the ground,” he said.
“I want to play as many games as possible and help the club have a successful season.”
Parkinson has been eager for cover in that area of the pitch after Laurens De Bock cut short his loan from Leeds United in the early stages of the window.
The Black Cats boss had praised both the temperament and quality in possession the Belgian possessed, but his time on Wearside had been stop-start after a long spell without senior football at Elland Road.
He wanted to move closer to Belgium and has subsequently joined up with Alan Pardew at Eredivisie side ADO Den Haag.
Denver Hume has thrived under Parkinson and the manager was eager that any new arrival could replicate the attacking threat the quick and direct 20-year-old has played with of late.
He had lined up one target early in the window and agreed the broad strokes of a deal, only for a failure to reach a new contract with his parent club scuppering the agreement.
His search then led to John, who has played five times this season.
He confirmed his interest after the 1-0 win over Tranmere Rovers, saying he could be a ‘real outlet’ for the Black Cats.