Pieters had a short trial period at Rockliffe earlier this month on the back of the defensive injury crisis at the Riverside, but Boro decided against offering the 36-year-old a deal.
Pieters then received a message from Derby boss John Eustace, and after another short trial period the Rams have signed the defender for the remaining eight games of the season.
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Pieters said: “Before I came here I trained with Middlesbrough, then the gaffer (Eustace) sent me a message and asked me my situation.
“It’s obviously a bit closer to home than Middlesbrough. I said I’m still a free agent and I’m ready to go.
“I trained with the lads and showed I’ve still got it! The gaffer was pleased with it and here I am.
“I still feel ready to play. At the moment I didn’t have any clubs lining up for me so I just started sending messages out to see what’s available. This is really good for me. It’s a really great group. They’re good fun and really determined in what they do.
“I think I can help the team with my experience. I’m here to play. But behind the scenes I will help everyone through these eight games. It’s really important to stay in the league.”
Pieters, holder of 14 Netherlands caps, arrived in English football when he joined Stoke from PSV Eindhoven in 2013, moved on to Burnley in 2019 and then spent the last two seasons with West Brom. He had a brief stint with Luton earlier in the season.
It is a second new addition for Derby under John Eustace, with forward Kemar Roofe arriving on a similar deal last month.
The Rams are currently fighting for survival in the Championship, lying in 22nd place, a point adrift of safety. But their hopes were boosted by three straight wins before the international break.
