The 60-year-old has Parkinson's disease which he said he had been working with "reasonably successfully", for the past six years.
However, the school had grown considerably over the past decade and he was finding it increasingly challenging to keep up with the required workload, he said.
"The school has grown and it's the number of hours that don't fit into what my body can do."
His official departure date had not yet been determined because he hoped to stay in the role as long as his health would allow, but he believed there would come a time "in the near future" when it would become untenable.
He was now starting work on passing over the reins, he said.
Board of trustees chairwoman Maree McDonald said Dr Galloway would be leaving the school in excellent shape.
"During Dr Galloway's tenure, we have grown in diversity and roll size, embarked on significant property projects and established innovative teaching and learning programmes.
"A strengths-based approach to learning and a focus on assisting children to develop good character has defined the education provided by the school over the last 15 years, and will be Dr Galloway's lasting legacy."
Dr Galloway's career began in 1980 at Watlington Intermediate, in Timaru. This was followed by teaching positions in Christchurch, Southland and Otago and as an adviser to schools for Massey University College of Education.
His first principalship was at Weston School, in North Otago, before moving to George Street Normal School in 2005.
Co-deputy principal Anne Robertson will retire at the end of this year, so fellow co-deputy principal Leanne Stanton will be acting principal when Dr Galloway steps down, until a new principal is appointed.
Dr Galloway said he had no plans for the future yet.
"At this stage, I'm fully focused on finishing here well, and I want to leave the school in the strongest possible position.
"So any plans for the future will wait until I'm finished here."
