diff --git a/NEWS b/NEWS
index 8262d1618e..5621c132fe 100644
--- a/NEWS
+++ b/NEWS
@@ -102,6 +102,9 @@ Changed Functionality
 - Parameter lists for functions, events and hooks now use commas instead of
   semicolons in error messages or when printing such functions.
 
+- The IO buffer size used for PCAP file reading is now always 128kb. This
+  new default can be changed via ``Pcap::bufsize_offline_bytes``.
+
 Removed Functionality
 ---------------------
 
diff --git a/scripts/base/init-bare.zeek b/scripts/base/init-bare.zeek
index 4e96175fda..c71faa7979 100644
--- a/scripts/base/init-bare.zeek
+++ b/scripts/base/init-bare.zeek
@@ -5286,6 +5286,11 @@ export {
 	## interfaces.
 	const bufsize = 128 &redef;
 
+	## Number of bytes to use for buffering file read operations when reading
+	## from a PCAP file. Setting this to 0 uses operating system defaults
+	## as chosen by fopen().
+	const bufsize_offline_bytes = 128 * 1024 &redef;
+
 	## Default timeout for packet sources without file descriptors.
 	##
 	## For libpcap based packet sources that do not provide a usable
diff --git a/src/iosource/pcap/Source.cc b/src/iosource/pcap/Source.cc
index c8869a0c7c..aa79935f87 100644
--- a/src/iosource/pcap/Source.cc
+++ b/src/iosource/pcap/Source.cc
@@ -10,6 +10,8 @@
 #include <pcap-int.h>
 #endif
 
+#include <stdio.h>
+
 #include "zeek/Event.h"
 #include "zeek/iosource/BPF_Program.h"
 #include "zeek/iosource/Packet.h"
@@ -176,10 +178,42 @@ void PcapSource::OpenOffline()
 	{
 	char errbuf[PCAP_ERRBUF_SIZE];
 
-	pd = pcap_open_offline(props.path.c_str(), errbuf);
+	FILE* f = nullptr;
+	if ( props.path == "-" )
+		{
+		f = stdin;
+		}
+	else
+		{
+		if ( f = fopen(props.path.c_str(), "rb"); ! f )
+			{
+			Error(util::fmt("unable to open %s: %s", props.path.c_str(), strerror(errno)));
+			return;
+			}
+
+		// Setup file IO buffering with a bufsize_offline_bytes sized
+		// buffer if set, otherwise use what fopen() took as the default.
+		if ( BifConst::Pcap::bufsize_offline_bytes != 0 )
+			{
+			iobuf.resize(BifConst::Pcap::bufsize_offline_bytes);
+			if ( util::detail::setvbuf(f, iobuf.data(), _IOFBF, iobuf.size()) != 0 )
+				{
+				Error(util::fmt("unable to setvbuf %s: %s", props.path.c_str(), strerror(errno)));
+				fclose(f);
+				return;
+				}
+			}
+		}
+
+	// pcap_fopen_offline() takes ownership of f on success and
+	// pcap_close() elsewhere should close it, too.
+	pd = pcap_fopen_offline(f, errbuf);
 
 	if ( ! pd )
 		{
+		if ( f != stdin )
+			fclose(f);
+
 		Error(errbuf);
 		return;
 		}
diff --git a/src/iosource/pcap/Source.h b/src/iosource/pcap/Source.h
index cbff27963a..ad696f1596 100644
--- a/src/iosource/pcap/Source.h
+++ b/src/iosource/pcap/Source.h
@@ -4,6 +4,7 @@
 
 #include <sys/types.h> // for u_char
 #include <unistd.h>
+#include <vector>
 
 extern "C"
 	{
@@ -44,6 +45,9 @@ private:
 
 	pcap_t* pd;
 	struct pcap_stat prev_pstat = {0};
+
+	// Buffer provided to setvbuf() when reading from a PCAP file.
+	std::vector<char> iobuf;
 	};
 
 	} // namespace zeek::iosource::pcap
diff --git a/src/iosource/pcap/pcap.bif b/src/iosource/pcap/pcap.bif
index e9ba0cbb5d..9d73cb402e 100644
--- a/src/iosource/pcap/pcap.bif
+++ b/src/iosource/pcap/pcap.bif
@@ -3,6 +3,7 @@ module Pcap;
 
 const snaplen: count;
 const bufsize: count;
+const bufsize_offline_bytes: count;
 const non_fd_timeout: interval;
 
 %%{
diff --git a/testing/btest/Baseline/core.pcap.input-error/output2 b/testing/btest/Baseline/core.pcap.input-error/output2
index bbbdf5b077..df60727de5 100644
--- a/testing/btest/Baseline/core.pcap.input-error/output2
+++ b/testing/btest/Baseline/core.pcap.input-error/output2
@@ -1,3 +1,3 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
 fatal error: problem with interface NO_SUCH_INTERFACE 
-fatal error: problem with trace file NO_SUCH_TRACE (NO_SUCH_TRACE: No such file or directory)
+fatal error: problem with trace file NO_SUCH_TRACE (unable to open NO_SUCH_TRACE: No such file or directory)
diff --git a/testing/btest/Baseline/core.pcap.wrong-format/output b/testing/btest/Baseline/core.pcap.wrong-format/output
new file mode 100644
index 0000000000..8129e9c851
--- /dev/null
+++ b/testing/btest/Baseline/core.pcap.wrong-format/output
@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+fatal error: problem with trace file not-a.pcap (unknown file format)
diff --git a/testing/btest/Baseline/core.pcap.wrong-format/output2 b/testing/btest/Baseline/core.pcap.wrong-format/output2
new file mode 100644
index 0000000000..e61dec2aac
--- /dev/null
+++ b/testing/btest/Baseline/core.pcap.wrong-format/output2
@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+fatal error: problem with trace file - (unknown file format)
diff --git a/testing/btest/core/pcap/wrong-format.zeek b/testing/btest/core/pcap/wrong-format.zeek
new file mode 100644
index 0000000000..c480dc1f92
--- /dev/null
+++ b/testing/btest/core/pcap/wrong-format.zeek
@@ -0,0 +1,10 @@
+# @TEST-REQUIRES: test "${ZEEK_USE_CPP}" != "1"
+# @TEST-EXEC-FAIL: zeek -b -r not-a.pcap >output 2>&1
+# @TEST-EXEC: btest-diff output
+# @TEST-EXEC-FAIL: cat not-a.pcap | zeek -b -r - >output2 2>&1
+# @TEST-EXEC: btest-diff output2
+
+@TEST-START-FILE ./not-a.pcap
+%PDF-1.5
+This isn't an actual pdf file, and neither a PCAP.
+@TEST-END-FILE