The host of the hit BBC motoring show was seen in Builth Wells in the Porsche 911 Targa 4S Heritage Edition, before stopping off at Bradley's Peugeot Garage in Llanidloes for petrol and a cup of coffee.
Fans of the show asked him for selfies, for which he was happy to oblige.
Gill Williams, forecourt manager of the petrol station, said: "This stunning Porsche came into the forecourt. We weren't sure it was him at first because he had a mask on, but we recognised him as soon as he took it off.
"We had a couple of selfies and chatted to him. He bought a couple of bits and bobs and had a coffee. He was lovely.
"I think he was meeting the other two, Freddie (Flintoff) and Paddy (McGuinness) in a forest to practice on gravel. It was just one of those moments when you're at the right place at the right time."
She said he was heading up past the Clywedog Reservoir to Staylittle before driving on to Machynlleth and Coed y Brenin Forest Park.
The £109,725 Porsche he was driving can do 0-62mph in 3.8 seconds, and has a top speed of 189mph.
Automotive journalist Harris joined Top Gear in 2016 after long-time favourites Jeremy Clarkson, James May and Richard Hammond left the series.
He originally co-hosted with Matt LeBlanc and Rory Reid, before Flintoff and McGuinness replaced them and joined him in 2019.