Fraser Stevenson, 35, has been left largely housebound after his black curly-coated cross retriever Tess vanished last month.
Mr Stevenson had let Tess off her lead at the River Nairn near his home in the Highland town for exercise on July 23. The river was at very low tide and she crossed over the other side.
Mr Stevenson said: "I made my way round the bridge to cut her off, by which time I think she had come back to where we originally were and carried on to the playpark and through the fields. For the next few hours members of the public went up and down the river looking for her, but there was no sign of her.
"By now most of us are of the opinion that somebody has taken her in. There are so many possibilities. There is the caravan park down the river and the main A96 road fairly accessible from the river."
Tess had been helping him for the past five years thanks to Guide Dogs Scotland. She is microchipped and wears a tag bearing the charity's name. Fraser added: "There is a guide dog instructor who has taken me out a couple of times and I have friends who help with the likes of shopping. I can take my other old dog out for a few minutes, but I haven't been able to do much without Tess."
Guide Dogs Scotland spokesman Niall Foley said "We believe Tess is with someone, who may have taken her in for safe keeping, with the best of intentions.
"Our primary concern is to find Tess so we can reunite her with Fraser, who is understandably distraught."