The 82-year-old woman was walking alongside a main road near Milton Keynes in the middle of the afternoon when the attacker came up behind her.
After the woman was sexually assaulted, the attacker – described as a large white man aged in his early 20s – walked off.
Police are now appealing for information about the assault and are looking to speak to a blackberry picker and dog walker spotted near the scene.
Investigator Samantha Arnold said: “We take all reports of this nature extremely seriously and I would like to reassure the local community that we are conducting a thorough investigation into this offence.
“I would like to speak to anyone who may have seen or heard anything unusual on the redway running alongside to Buckingham Road, behind Windmill Hill Drive, on Friday afternoon.
“I would particularly like to speak to a man who was walking a dog, believed to be a white dog, possibly a husky, along the redway prior to the offence.
“I would also like to speak to a man, about 60 years old, seen picking blackcurrants near to the scene.
“The victim is being supported by Thames Valley Police following this horrendous incident.”
The alleged assault is believed to have happened at 1pm on Friday.
The suspect was wearing a black hat, black clothes and carrying a black rucksack and walked of in the direction of Bletchley following the attack.
Anyone with information is asked to call 101 or Crimestoppers anonymously on 0800 555111.