diff --git a/costools/add_cos_s_region.py b/costools/add_cos_s_region.py
new file mode 100644
index 0000000..4411cb2
--- /dev/null
+++ b/costools/add_cos_s_region.py
@@ -0,0 +1,131 @@
+#!/usr/bin/env python
+
+# Add the S_REGION keyword to raw files
+#
+import os
+import glob
+import argparse
+
+from calcos import cosutil
+from astropy.io import fits
+
+def main(rootnames, dry_run=False):
+    """Add the S-region keyword to COS FITS files in the current
+    directory that meet the following criteria:
+
+    ['rootname_rawtag.fits',
+     'rootname_rawtag_a.fits',
+     'rootname_rawtag_b.fits',
+     'rootname_rawaccum.fits',
+     'rootname_rawaccum_a.fits',
+     'rootname_rawaccum_b.fits',
+     'rootname_rawacq.fits']
+
+    The S_REGION keyword will have the value
+    'CIRCLE ICRS ra_aper dec_aper diameter'
+    where ra_aper and dec_aper are in decimal degrees and are read from
+    the extension 1 header, and diameter = 2.5/3600.0, which is the diameter
+    of the PSA and BOA in decimal degrees.
+
+    The keyword will be added after the PA_APER keyword in the extension 1
+    header
+
+    """
+    if rootnames is None:
+        cosutil.printError("No rootnames specified")
+        return
+
+    files_to_process = get_files_to_process(rootnames)
+    for input_file in files_to_process:
+        add_s_region(input_file, dry_run)
+    return
+
+def get_files_to_process(rootnames):
+    """Create a list of files to process from the list of rootnames
+
+    """
+
+    endings = ['rawtag.fits',
+               'rawtag_a.fits',
+               'rawtag_b.fits',
+               'rawaccum.fits',
+               'rawaccum_a.fits',
+               'rawaccum_b.fits',
+               'rawacq.fits']
+    file_list = []
+    for rootname in rootnames:
+        if os.path.basename(rootname) != rootname:
+            cosutil.printWarning("{}: rootnames should refer to files in the working directory".format(rootname))
+        fitslist = glob.glob(rootname.lower() + '*.fits')
+        appended = False
+        for input_file in fitslist:
+            for ending in endings:
+                if input_file.endswith(ending):
+                    appended = True
+                    file_list.append(input_file)
+        if not appended:
+            cosutil.printWarning("{}: No files found for this rootname".format(rootname))
+    return file_list
+
+def add_s_region(input_file, dry_run):
+    """Add the computed S_REGION to the keyword
+    """
+
+    open_mode = 'readonly'
+    if not dry_run: open_mode = 'update'
+
+    with fits.open(input_file, mode=open_mode) as f1:
+        prihdr = f1[0].header
+        if prihdr['INSTRUME'] != 'COS':
+            cosutil.printWarning("File {}: Instrument is not COS".format(file))
+            return
+        success = False
+        for n_ext, extension in enumerate(f1[1:]):
+            exthdr = extension.header
+            extname = exthdr['EXTNAME']
+            extver = exthdr['EXTVER']
+            if extname in ['SCI', 'EVENTS', 'ACQ']:
+                try:
+                    ra_aper = exthdr['RA_APER']
+                    dec_aper = exthdr['DEC_APER']
+                    pa_aper = exthdr['PA_APER']
+                except KeyError:
+                    cosutil.printWarning('One or more of RA_APER, DEC_APER, PA_APER is not present')
+                    cosutil.printContinuation('in extension {} header of file {}'.format(n_ext+1, input_file))
+                    continue
+                diameter = 2.5 / 3600.0
+                s_region = 'CIRCLE ICRS {0:.8f} {1:.7f} {2:.8f}'.format(ra_aper, dec_aper, diameter)
+                if not dry_run:
+                    try:
+                        existing_s_region = exthdr['S_REGION']
+                        exthdr['S_REGION'] = s_region
+                        cosutil.printMsg("Using existing S_REGION keyword in {}[{}, {}]".format(input_file, extname, extver))
+                    except KeyError:
+                        exthdr.insert('PA_APER', ('S_REGION', s_region, 'Spatial extent of the observation'), after=True)
+                        cosutil.printMsg('Added S_REGION keyword to {}[{}, {}]'.format(input_file, extname, extver))
+                    cosutil.printMsg("Setting value to '{}'".format(s_region))
+                else:
+                    cosutil.printMsg("S_REGION for {}[{}, {}] = '{}'".format(input_file, extname, extver, s_region))
+                    cosutil.printMsg("Dry-run - no changes made to science header")
+                success = True
+        if not success:
+            cosutil.printMsg('No S_REGION keyword added to file {}'.format(input_file))
+
+def call_main():
+
+    parser = argparse.ArgumentParser(
+    """Add S_REGION value to raw data headers"""
+    )
+
+    parser.add_argument('rootnames', nargs='+',
+        help='Rootnames to be processed')
+
+    parser.add_argument(
+        '--dry_run', action='store_true',
+        help="Calculate S_REGION value, but don't write to data header[s]")
+
+    args = parser.parse_args()
+    main(args.rootnames, dry_run=args.dry_run)
+
+if __name__ == "__main__":
+    call_main()
diff --git a/setup.py b/setup.py
index 9b3e319..59a590c 100755
--- a/setup.py
+++ b/setup.py
@@ -36,8 +36,12 @@
     entry_points={
         'console_scripts': [
             'timefilter = {0}.timefilter:main'.format(PACKAGENAME),
+            'add_cos_s_region = {}.add_cos_s_region:call_main'.format(PACKAGENAME),
         ],
     },
+    scripts=[
+        'costools/add_cos_s_region.py',
+    ],
     author='Warren Hack, Nadezhda Dencheva, Phil Hodge',
     author_email='help@stsci.edu',
     description='Tools for COS (Cosmic Origins Spectrograph)',