The 66-year-old man, who has not yet been named, was travelling uphill along Pleasley Road towards Aughton when his black Mazda 3 was involved in a head-on collision with a grey Mercedes travelling downhill towards Whiston.
The collision occurred near the entrance to Ulley Country Park just before 8am.
The Mazda driver was pronounced dead at the scene while the 57-year-old man behind the wheel of the Mercedes was taken to hospital with a stomach injury.
He remains in hospital today.
In a fresh appeal for information about the collision, South Yorkshire Police said: "Traffic officers are appealing for witnesses and dash-cam footage after a man and his dog died in a collision in Rotherham yesterday.
“Shortly before 8am, a black Mazda 3 was travelling uphill on Pleasley Road towards Aughton, when it was in a collision with a grey Mercedes travelling down the hill in the opposite direction.
“Sadly, the 66-year-old man driving the Mazda died at the scene. His dog, who was travelling in the back of the car at the time of the crash, also died.
“The driver of the Mercedes, a 57-year-old man, is currently in hospital recovering from an injury to his stomach.
“Police are keen to hear from anybody who was in the Aughton area yesterday morning and witnessed the collision, or the manner of driving of either vehicle prior to this.
“We are particularly keen to hear from anyone who captured either vehicle on dash-cam footage or CCTV.”