Under half of S2 pupils performing well in writing, down from 64% in 2012
The findings have been revealed by the Scottish Survey of Literacy (SSLN) and Numeracy.
Less than half of Scotland's 13 and 14-year-olds are performing well in writing.
The SSLN looked at performance in school years P4, P7 and S2, focusing on literacy in 2016.
It found 49% of S2 pupils performed very well or well in writing last year, compared with 55% in 2014 and 64% in 2012.
P4 and P7 pupils also saw a drop in writing performance, while reading performance across all three age groups was broadly stable.
Education Secretary John Swinney said the figures were ''simply not good enough''.
Among P4 pupils, 77% were performing very well or well in reading, down from 78% in 2014.
In writing, performance dipped from 64% to 62%, but in listening and talking it rose from 59% to 60%.
In P7, the figure for reading stood unchanged at 88%, while in writing it dropped from 68% to 65%, and in listening and talking performance rose from 66% to 67%.
In S2, reading performance rose from 80% to 82%, and listening and talking dipped from 52% to 49%.
The attainment gap - the difference in performance by Scotland's most and least deprived pupils - remained broadly similar last year to 2014 when it came to literacy.
Speaking during a visit to Craigroyston Primary School in Edinburgh, Mr Swinney said: ''I want to see standards and attainment improving in Scottish education. A stable performance and drop in S2 writing is simply not good enough.
There are four key areas where we need improvement. We need to better understand the progress of individual pupils, be clear about the standards expected in our classrooms while stripping out bureaucracy to free teachers to teach, and ensure literacy skills are fully embedded across the curriculum.
This Government is already taking action to provide teachers and schools with the tools - through the literacy benchmarks and standardised assessments - and the resources, through the Scottish Attainment Challenge and Pupil Equity Funding, to improve literacy.
These reforms are not an overnight solution, it will take time before we see their full effect and we must stay the course.
But if anyone looking at these literacy results thinks nothing more needs to change in Scottish education then they are mistaken. Further reform is now imperative.''